So if I wanted to make the best defensive character I could think of, what would be the priority of features to give them the best survivability?
Defensive features to consider:
Armor Class.
Hit Points.
Disadvantage on attacks/effects against you.
Damage Reduction or Immunity.
Obviously if you could have them all that would be best, but how would you prioritize them?
I'm thinking: Disadvantage > AC > DR > HP. What do you guys think?
Are there any other general features to consider that you think I missed?
Depends on how much, and at what cost. +5 AC > Disavantage, but Disavantage > +1 AC.
Also, DR when you're hit 90% of the time is much nicer the DR when you're hit 10% of the time.
Immunity is obviously the best. But i'm not sure you can even get it, let alone the right type at a good cost.
Disvantage (or advantage for saves), is just great all around, but tends to not be cheap.
Gnomes are the obvious, but don't discount halflings lucky.
Half-orcs also have a strong defensive feature.
Cleric 1 gives you heavy armor and shields.
Paladin 6 (+Cha) or monk 14 net you alot of saves.
The only general advice i have is...
+2 Con = +1 AC for weapon attacks.
+2 Con > +1 AC for everything else.
Overall Disvantage = Resistance. Since both will 1/2 the damage taken.
Disavantage > Resistance for weapon attacks (due to crit), and control spells.
Resitance > Advantage for 1/2 damage spells.
You're forgetting saves, and the meta-defense of having the option to sneak past something that would otherwise kill you.
I'd go with a Halfling dexterity Paladin as being the toughest PC to kill in the game. Studded leather and 20 Dex is just one AC point away from best available. Great Stealth. Reroll auto failures on saves. Add Charisma to saves at level 6+. Selfish application of Lay on Hands is essentially +10 to Constitution.
Close second would be a Human Barbarian with Medium Armor Mastery. Half plate and shield. But you're going to get hosed vs spells.
Wood Elf seems like it would be a better option for stealth. The tank hiding behind the wizard seems counter productive. It also is a lot more mobile letting you pick your position better and of course, letting you escape easier when things go south.
Halfling luck is great though, I can't argue that 
Bear rage works in heavy armor.
A mountain dwarf totem with heavilied armored + heavy armor mastery would be extreamly difficult to kill.
This talk of barbarian's using shields got me thinking about shield mastery too. That talent has a lot of synergy with barbarians. Advantage on the bonus action shove. Advantage on the reaction dex save for zero damage. +2 bonus to dex saves.
I'm not sure how effective that shove really is though from a defensive perspective, even with advantage and athletics.
Demosthenes2054 wrote:
Uncanny Dodge is particularly good at reducing damage, takes level 5 Rogue or high level Ranger though.
Resilience Feat is good for a primary save that you don't get from your class, especially if adds to DEX or CON as either of those increase your AC or HP respectively.
Uncanny Dodge would stack with DR, right? Say you get hit for 20 points, you use you're reaction to halve it to 10, the DR takes it down to 5?
What would be save priority?
Levels in Barbarian are a definite must. Is the +Cha to saves enough to go Paladin? Uncanny Dodge and Evasion would definitely be good.
Wouldn't 20 Dex & 24 Con Barbarian with a shield net you the highest non-magic AC at 24? With the right magic items (Ring of Protection and Bracers of Defense) and Dual Wielder you would have a 27 AC. Now throw Haste on them for good measure...
There are no auto failures on saves.
Bear Totem resistance with heavy armor is debatable.
Stats cap at 20 for players.
Bracers of Defense won't stack with barbarian AC.
So 22 AC with 20 Con, 20 Dex, and a shield. But unless you're rolling stats or using magic items to boost stats you're blowing four feats to get that.
Danny_Montanny wrote:
Ahh..
Well as long as we're theorycrafting high level builds a level 18 Wizard can cast Shield on every reaction. A dip in a martial class gives that same Wizard Heavy Armor and Shield Prof. That's 25 AC. Foresight gives everyone disadvantage when trying to hit you and you advantage on all saves. Couple with Abjurer and you have resistance to all spell damage and your Abjuration Ward gains at least 2 hit points each turn (from Shield).
That's just off the top of my head. I imagine the reality is much worse.
Danny_Montanny wrote:
Demosthenes2054 wrote:
Rancid_Rogue wrote:
mellored wrote:
Danny_Montanny wrote:
I am a fan of Paladin 6, Monk 14 with 20 cha and proficiency in all saves while wearing the heaviest armor. You can also use the dodge action 14 times per short rest as a bonus action.
The halfling paladin posted above seems like the most realisitc party tank if that's what someone is going for. The armored barbarian options look nice too.