It seems to me that you'd need a high intelligence and a way to get many knowledge skills as class skills, or you'd have to be in a game with only a limited number of foes. An INT based gish seems a likely bet, or a warblade who only ever fights humanoids.
But speaking more generally, assuming you can't afford to spend skill points on covering every possible creature type, which skills do you think are the "best" for it?
Local (humanoids) seems like an excellent choice if you're in an urban game.
Acana covers dragons, magical beasts, and constructs, another good selection.
Planes is obvious for outsiders.
Nature covers giants, animals, plants.
Dungeoneering covers aberrations.
Religion for undead.
But really, can you make a build that has enough skill points to spare to cover all of these and still be a strong melee combatant? If you can't cover all of them, which do you think are the best 2 or 3?
I think maybe the factotum would be a good build for that; INT is his most important attribute, and he gets all knowledge skills (all skills period) as class skill, and he has 6+ skill points. problem there is, he has average BAB. but you can make up for that with clever use of inspiration points.
*EDIT: all keep in mind the Master of Knowledge feat from HoH. It provides a +1 to all knowledge checks. and that's a potential +1 to hit and damage against any creature you have a rank of knowledge in.
-Draconic knowledge least invocation for the dragonfire adept gives +6 DM p.80
-the gnomes from dragonlance campaign setting if you choose sage guild you get +2 DLCS p.26ish
With Holy warrior and Knowledge devotion alone they can have +5 to hit and +14 to damage.
- but if you are a cleric why not get law devotion 
-The already mentioned
-Archivist (any build that cares about attacking, say Archivist Archer; Int-based 4+Int class with all knowledges)
-Eternal Blade (won't have +5s though, but Eternal Knowledge gets you nice +2/+3 with exactly 0 skill investment)
Also, some Wizard Gishes could do decently if they happen to have Able Learner or Education around; generally, anything Int-based that doesn't really need as many skillpoints to cover other things. It's a bonus if you're expected to pick Knowledges anyways, such as with an Archivist.
Getting bonuses is pretty easy. Gnomes can take the Trivial Knowledge feat, which lets them reroll knowledge checks... very nice. Scrolls of Uncertain Provinence gives you a +5 to knowledge checks for 8kgp if you have the right alignment, while a Tome of Worldly Memory can be very handy for just 1500gp if you're willing to spend a standard action (during the surprise round or if you saw the enemy first), though that works only 3 times per day. Don't forget Masterwork Tools from the PHB... 50gp can give you a +2 (I call them Masterwork Field Manuals). Also, Collector of Stories will grant a +5 bonus on one enemy type per encounter, at a cost of two skill points. Those things combined would give a +17 bonus to knowledge checks with a reroll for a total cost of one feat, one skill trick, and a bit under 10kgp. That's quite handy... it's a +3 or 4 to hit and damage with all attacks, in addition to knowing everything you need to know about everything you're fighting. VERY potent.
Generally speaking, Knowledge Devotion is going to be best on people who make a lot of attacks, and that tends to mean archers or TWFers. I find it's a great feat for a field commander type character... stand back and shoot while telling your allies exactly how to attack and what the vulnerabilities of the target are.
One example field commander character might be a Binder 1/Archivist 4/Anima Mage 10/Tenebrous Apostate 5. Bind Dantallion and some other vestige, use Persistant Lore of the Gods, and go nuts. You should have no trouble hitting the necessary DCs to give yourself +5 to hit and damage, while using Dark Knowledge (Tactics) to give all allies +3 to hit, all the while making sure your allies know exactly what the creature is capable of (and probably its mother's maiden name). Persist other goodies and make yourself into a great archer while you're at it... I recommend Divine Power, Righteous Wrath of the Faithful, Vigorous Circle, and Recitation.
As for how to make Cloistered Clerics potent, I made an archer build that persisted Divine Power, Lesser Holy Transformation, Divine Favor, Righteous Might, and Guided Shot (via Anyspell). I was persisting something else too (21 turn attempts, using extend so the spells last 48 hours. I cast three of them per day). Then I used Chain of Eyes and Greater Magic Weapon. Result? I can see through the eyes of the party scout while flying half a mile above. If he sees a target, I take it out. The target is unlikely to spot the scout, while they have a -200 to spot me. Yehaw. Meanwhile, I'm doing pretty darn good damage... Strongarm Bracers of Archery give me proficiency with all bows and let me use larger ones. A Huge Footbow becomes a great weapon in this situation, since I have a nice high strength score, and obviously Knowledge Devotion rocks too.
JaronK
Wizards have Int as their primary stat and they'll easily take 2-3 Knowledge skills. Of course, you should build your Wizards towards this (Ray casters or Gishes).
The type of Knowledge depends on your campaign.
IMO, In order of ranking:
1. Local (humanoids) - excellent *if* your DM rules all humanoids for this check. Not all DMs will take this interpretation the usage of "local" as a description makes it tricky.
2. Arcana - because it synergizes with Spellcraft for casters.
3/4. Dungeoneering or Religion. Undeads are tricky to deal with most of the time, so good to have. Dungeoneering usually because you end up in Dungeons.
5. The Planes is excellent if your campaign is set up with a lot of outsiders. Otherwise, only good to have.
6. Nature creature encounters are a little rarer to justify taking.
For the record, my Wizard character doesn't have Knowledge local because 1. I doubt my DM is assuming all humanoids, 2. Humanoids are easier to handle than monsters.
http://forums.gleemax.com/showthread.php?t=1133088
Doing what I do best... *wink*
Anyone relying on enervation or ray of stupidity for their primary spell.
Go reread the section in Complete Arcane on Weaponlike Spells. Bonus damage stacked on to spells that don't do HP damage is typically considered negative energy damage. So, your Enervation wouldn't do 1d4+5 negative levels, it would do 1d4 negative levels, +1-5 negative energy damage.
Same as if you tried to Sneak Attack with Enervation.
What about Shadows or Allips? There's a thread on optimizing their ability damage and the author makes a good case on the application of such mundane bonuses applying to ability damage as ruled by the RAW. Search in SHCOL.
I dunno mang, all I'm saying is thems the rules for Weapon Like Spell, as presented in Complete Arcane. The specific example of using Point Blank Shot in conjuncture with something is even given. Don't have CA on me, anyone care to cite it?
As far as undeaders go, I'd say it would be incredibly silly/op to allow them to use SA to increase ability damage or level drain. I mean, a Shadow is CR3. A Rogue 3 Shadow would be CR6, unless Rogue would be considered to be a non-associated class for Shadows, then it would be CR4 (rounding up). Lets just call it CR6, which makes 1 an EL6 encounter, suitable for a party of 4 characters about level 4-5. Our shadow has a +5 to hit, unless it was hiding, which it does well, giving it a +7. Its attacking touch AC, or flat footed touch AC if it was hiding, which would be rediculously easy to hit. Now, its rolling 3d6 for str damage, unless....we give it Craven. Now 3d6+6 averages 16, which means happy happy spawn time for the entire party, except for maybe the barbarian, and only if he rages, and then the shadow could still get him on a good roll...probably before the party even realize its there.
Now, I don't know what RAW would say about this, but if I ever tried to pull one of these on my players, I'd probably get pushed out into traffic, and my players actually LIKE me. I'd highly recommend against this, whatever the case. I'd rather put SA on a Ghoul, which are very sneaky according to their description. Then you just automatically get SA on every hit after they fail a fort save, due to paralysis. Dangerous, but not an insta-pwn.
EDIT:
Just realized undead can't take Craven, but still, 3d6 will 1 shot anyone without str as a prefered stat MOST of the time...
FAQ (pg 47 in Red) also supports this, take that as you will, indicating
"Feats and other sources that give bonuses to damage do not affect ability damage unless they specifically say so."
Taking the Wyvernhands 3 Rogue/Shadow example it would be
1d6 Str + 2d6 SA "negative energy" damage
Multiclass with ranger and one can get 5 knowledge skills as class skills (2 from janissary, 2 from ranger, 1 from knowledge devotion) while maintaining full bab progression.
edit: When selecting sources of bonus to knowledge check, remember that a lot of stuff doesnt stack. Scrolls of uncertain provenance and collector of stories both provide the same type of bonus (I think it is insight bonus), and so do not stack. Fortunately either one of them will stack with lore of the gods (If I recall it provides inherent bonus).
6. Nature creature encounters are a little rarer to justify taking.
Per the PH, Nature covers: Giants, Fey, Monstrous Humanoids, Plants, Vermin, and Animals. Some DM's also include humanoids not covered by 'local' in that as well.
That's not a bad list, it's the most types of creatures for one skill available (even without humanoids), at 6 types (compared to 3 from Arcana), and they aren't all that uncommon to encounter (at least, not in my experience).
Obviously, if your DM doesn't throw any of those at you, it's not worth it, but that's 43 entries in the MM 1 (and that's counting Animals, Dinosaurs, Dire Animals, Giants, Giant Insects and the like as one entry each). Arcana, by comparison covers 56 entries in the same book, but only Dragons and Golems have multiple sub-entries.
YMMV on which is better for your campaign/character, but I wouldn't say it's unjustifiable.
I used knowledge devo on my first cleric... i'll never use it again. It's a good power up, better than most, but clerics get the best buffs in the game, and some of the best spells, if you use this... I don't want to say that you'll be wasting a turn... but... I guess what i'm trying to say is that there are certainly more valuable things you could be doing with your turn and if you set the bar for what buffs are worth the turn they take to cast low enough to include this, than you may spend all of combat buffing and never make use of your supreme strength.
Duskblade would be a perfect fit for this, if you stack int, like you should, then you shouldn't have any problem making this work.
It seems like that alone makes the feat a worthwhile investment for any warrior who can spare a handful of skill ranks.
I mean a Human Warblade who manages 12 or 14 int should be able to max out two knowledge skills (Local and one of the player's choice) and easily drop a few cross class ranks in others to get a +1 attack and damage.
Seems like that alone may be worth it.
Per the PH, Nature covers: Giants, Fey, Monstrous Humanoids, Plants, Vermin, and Animals. Some DM's also include humanoids not covered by 'local' in that as well.
I agree it's campaign based. For the record, playing as a Conjurer wizard, "Local" is my 6th choice in my current campaign.
I put my ranking up based on the following criteria:
1. General frequency of encounters (most campaigns are humanoid based)
2. Cross related synergy
3. Is it essential to have a good defence against (type of creature)?
Lizardfolk: 2
Locathah: 2
Those are from the MM. Dunno about anywhere else.
+1 for duskblade
I used knowledge devo on my first cleric... i'll never use it again. It's a good power up, better than most, but clerics get the best buffs in the game, and some of the best spells, if you use this... I don't want to say that you'll be wasting a turn... but... I guess what i'm trying to say is that there are certainly more valuable things you could be doing with your turn and if you set the bar for what buffs are worth the turn they take to cast low enough to include this, than you may spend all of combat buffing and never make use of your supreme strength.
knowledge devotion does not take an action.
knowledge devotion does not take an action.
Yes, but certain ways of boosting the bonus do take a standard action.
And generally, if you're using the Tome you've got a character that can see the enemy first, then engage... giving time to use the Tome before combat actually starts.
JaronK
Getting bonuses is pretty easy. Gnomes can take the Trivial Knowledge feat, which lets them reroll knowledge checks... very nice. Scrolls of Uncertain Provinence gives you a +5 to knowledge checks for 8kgp if you have the right alignment, while a Tome of Worldly Memory can be very handy for just 1500gp if you're willing to spend a standard action (during the surprise round or if you saw the enemy first), though that works only 3 times per day. Don't forget Masterwork Tools from the PHB... 50gp can give you a +2 (I call them Masterwork Field Manuals). Also, Collector of Stories will grant a +5 bonus on one enemy type per encounter, at a cost of two skill points. Those things combined would give a +17 bonus to knowledge checks with a reroll for a total cost of one feat, one skill trick, and a bit under 10kgp.
Just to point out here that Collector of Stories doesn't grant a bonus to knowledge checks, it grants a bonus to knowledge checks made to "identify a creature or learn its special powers or vulnerabilities." JaronK might disagree with me, but I don't see it being relevant to Knowledge Devotion.
Which isn't necessarily a problem in any event as Collector of Stories, Scrolls of Uncertain Provenance, and Tome of Worldly Memory all provide a competence bonus.* Note also that to use a Tome of Worldly Memory as a standard action, you need to have five ranks in the knowledge skill you're using.
So really, those things combined would be overkill. But, you can get a +7 bonus to knowledge checks with a reroll for a cost of one feat, and either (a) a skill trick (which may or may not work and can only be used once/encounter anyway), or (b) a 1.5K magic item which can be used three times per day and requires either one minute or five ranks in the relevant knowledge skill and a standard action to use, or (c) an 8K magic item for which you need to be lawful.
As JaronK says, probably the best thing is just to use Divine Insight (which, err... does take an immediate action). Divine Insight: making skillmonkeys irrelevant since 2005!
*At least, in my printing of MIC, Scrolls of Uncertain Provenance provide a competence bonus. I'm not sure where Tshern is getting his information.
Just to point out here that Collector of Stories doesn't grant a bonus to knowledge checks, it grants a bonus to knowledge checks made to "identify a creature or learn its special powers or vulnerabilities." JaronK might disagree with me, but I don't see it being relevant to Knowledge Devotion.
I guess that depends on what you think the Knowledge Devotion check is. Personally, I'd say it's a check to see if you can identify your enemy and learn its vulnerabilities. After all, isn't that exactly what it is? What else could it be?
And yeah, Divine Insight is immediate, but it doesn't use up your ability to attack in the surprise round, which was the main point.
JaronK
I guess that depends on what you think the Knowledge Devotion check is. Personally, I'd say it's a check to see if you can identify your enemy and learn its vulnerabilities. After all, isn't that exactly what it is? What else could it be?
So I look at the SRD and see that one use of the knowledge skill is to "identify monsters and their special powers or vulnerabilities." Said use of knowledge skill has a DC equal to 10 + the monster's HD and provides one bit of useful information, and one more for every five points by which the check exceeded the DC. Of course you know all this, but we might as well be precise.
Since the Collector of Stories skill trick applies to knowledge checks made to "identify a creature or learn its special powers or vulnerabilities," and the two are almost direct quotes of each other, it's this check that I think Collector of Stories applies to.
I recognize that one could argue that Knowledge Devotion is a check to see what one knows about a given enemy that is useful in combat, as represented by a bonus to hit/damage. But the near direct quote suggests to me that Collector of Stories applies specifically to the check described in the PHB description of the knowledge skill. Certainly we wouldn't say that the Knowledge Devotion check tells one, for example, that the weird thing not looking at you is a Grimlock, and is immune to gaze attacks!
As always, of course, YMMV.
And yeah, Divine Insight is immediate, but it doesn't use up your ability to attack in the surprise round, which was the main point.
Just so. Just wanted people to remember that using Divine Insight does preclude using other swift/immediates.
Obviously, Collector of Stories is intended to be for the check to identify what you're fighting... but that check is pretty much the same as the one for Knowledge Devotion if it's made at the start of combat, and nothing in Collector of Stories indicates that it wouldn't work with Knowledge Devotion... I'd say it sure looks like it does work. I mean, how else would you describe the Knowledge Devotion check?
JaronK
I dunno, we've always used those two checks (the one to identify the critter, and the knowledge devotion check) as being one and the same as long as you're doing it at the start of combat.
Well, that may be part of the difference - I've always rolled twice!
Obviously, Collector of Stories is intended to be for the check to identify what you're fighting... but that check is pretty much the same as the one for Knowledge Devotion if it's made at the start of combat, and nothing in Collector of Stories indicates that it wouldn't work with Knowledge Devotion... I'd say it sure looks like it does work. I mean, how else would you describe the Knowledge Devotion check?
I certainly agree that it can be read that way; I don't see that it must be read that way. Certainly, since I use one Knowledge Devotion check for all creatures of a given type, it's not a particularly creature-specific check, so I can't think of it as telling me "well, this guy's a chain devil, and there's a weakness in his armor here, while this guy... well, I don't have any idea what he is, but the weakness in his armor his there."
I suppose it's a "rules don't say I can't" versus a "rules don't say I can" sort of argument, and I prefer to err on the side of reading the rules as telling me what I can do.
All of that's sort of missing the point, though. The basic point is that before applying Collector of Stories to the Knowledge Devotion check, check with your DM. Also that, given the choice between Collector of Stories, Scrolls of Uncertain Provenance, and Tome of Worldly Memory, the scrolls are probably your best best since you can use them as often as you want and it's very clear that they work. And lastly that you do have to make the choice between those three approaches, since they all give the same type of bonus.
There really is no Knowledge devotion check, it's Knowledge Devotion which requires a Knowledge check to benefit from. ( just the way we see it )
Part of the theory behind that is to prevent conflicting results.
Ex: Party Fighting...will use the Chain Devil just mentioned
Player 1: Rolls Knowledge check for Kn devotion, does well and gets + 5 damage now thanks to roll, skill ranks, and Collector of Storie's
Group: Asks what vulnerabilities Devil has.
Player 1: makes another check, rolls lousy...has no idea of weaknesses.
That result just didn't work for us and and the reverse would be just as odd.
KDev check provides little benefit +1
Knowledge check rolls well ( w/ CoStories) and player relates every weakness the Devil has to the party.
Note it's still possible, because the Kn Dev is based on Kn Check compared to Kn Dev chart outcome, where as the base Knowledge check part takes into consideration the creature HD vs that roll/skill result. It's just that the overall, a single check provides a more consistent comparison between the feat benefits and skill check benefits.
It just made more sense to us that way.