Now that many of us are finally getting an opportunity to look through our shiny new Monster Manuals, I thought it might be fun to see what everyone's top 10 favourites were. 
I will start things off with my top 10 list:
Dryads, Fire Giants, Goblins, Medusae, Merfolk, Orcs, Pixies, Pseudodragons, Skeletons, and Yuan-ti
Looking forward to seeing the monsters you love best. 
EDIT: Favourites could be chosen for the artwork, lore/background, fun to use in adventures, a combination of factors, etc. 
Depending on the attributes that make it a favorite... art? Interesting lore? Fun to use in combat?
Can't comment on the second two, as I haven't read cover to cover, but for art:
Cockatrice, Faerie Dragon, Genie - Efreeti, Hydra, Kraken, Merrow, Mummy, Pixie, Revenant, Slaad - Death Slaad,
OP (or someone other than me) should keep a tally to see which monsters get the most likes. I predict the adorable pixie will win.
In no particular order:
The entire beholder category.
Vampire.
The new approach to demons and devils (meaning less of a laundry list of spell-like abilities, but still plenty of "ouch" to throw around).
The new approach to dragons.
It's not a monster in particular, but it counts: the fact that life drain is now a dangerous temporary effect, rather than a "I'm done playing this character" inducingly unfun thing stopping me from using numerous monster types at all.
Off the top of my head, I love the beholder, lich, spellcasters in general (NPC or monster types), Oni, Hags, Wraiths and Wights, Medusa, Golems, and many more.
I love how energy drain takes off Max hit points until long rest or greater restoration. That really scares the heck out of my players.
What I find so interesting about the choices shared is that it provides a peek into the kinds (and moods) of campaigns/settings each of us prefers.
For my own selections, they lend themselves to:
Forest Environments: Dryads, Medusae, Pixies, Pseudodragons, Yuan-ti (Jungle)
Ruins: Goblins, Medusae, Orcs, Skeletons, Yuan-ti (Old Temples)
Seaside Environments: Merfolk, Pseudodragons (Nearby Woods and Caves)
Mountain/Volcano Environments: Fire Giants, Pseudodragons (Caves or Nearby Woods), Skeletons
Brown bears, Githyanki monks, hippogriffs, giant eagles, giant vultures, imps, giant wasps, swam of quippers, Githyanki warriors, giant elks, and polar bears.
At level, they're some of the hardest monsters in the game. The brown bear can put out enough damage to damn-near kill four 1st level fighters a round.
Additional: quadrones.
Art-wise; pretty much everything other than the owlbear. I really loved the new bone devil, cloaker, doppelganger, and carrion crawler, for example. They finally don't look like slightly goofy cartoons. I'm afraid of them now. The new centaur is bad-ass too.
Stat-wise; hands down the new Lich. I'm so glad it's actually REALLY powerful now. Mid-level Liches never seemed right.
Dragon turtle was my favorite art. Still on the pages that have CR 0 creatures (and working my way up), so no favorite monsters yet. I like the idea behind the new shadow dragons and hope they have plans for other environmental templates for dragons.
Mechagamera wrote:
The Gibberling
The Cave fisher
The Barghest
The Wemic
The xvart
Ya, those were awesome. Thanks wotc. 
Sorry, I gotta add this...
My wife and I have been flipping through, mainly checking out the art.
So many of these creatures are a 'gritty re-boot', LMAO.
If you can, check out the sprite. Right now. He's seen some S#!+, man. Looks like he was in 'Nam.
The Centaur also looks like some crazy goth killer, and not the bearded half horse renaissance faire-goer.
I think that the two best things about this monster manual are the writing and creative mythology of eac monster, and the art. Those two aspects make it a contender for *the best mosnter manual ever*. It is at least equal to the 3.0 monster manual, which was also awesome and very inovative.
My top ten for the moment:
Beholder (they seem more terrifying than in 3rd edition, love their random deadliness and lair powers)
Demi-Lich (great background, now I see how a demi-lich can be both 'demi' and more dangerous than a lich)
Aboleth
Modrons (I am very happy that this player's handbook has embraced the whimsy)
Succubus/Incubus (great that they can work for a variety of fiendish overlords)
Koa-Toa (I love that they invent weird gods to worship, and the illustration makes them look like hapless fan-boy deep ones)
Hags
Displacer Beasts, and vicariously, Blink Dogs (I like the backstory regarding the legendary displacer beast/blink dog animosity)
Satyr (the sonnet of a naughty satyr)
Troll (loathsome limbs)
Brightmantle wrote:
AaronOfBarbaria wrote:
Brock_Landers wrote:
AaronOfBarbaria wrote:
Brock_Landers wrote:
AaronOfBarbaria wrote:
Well, it looks like Brock completely smashed that argument to bits. heh.
AaronOfBarbaria wrote:
I'm familiar with MMs back through 2e and some of the lore seems brand new to me and is pretty cool. The thing about the first first harpy, for example, gives a whole level of flavor that wasn't there before. The kraken got an overhaul and while i like them as aberrations in 4e, the underlying titan lore that comes with some of these monster works well too.
From memory -
Weretiger
Chimera
Cambion
(I really love the Gith entry picture with the cities in the astral plane)
Mindflayer
PS: does anybody else find it weird the only full page picture is of the Myconid mushrrom people?
In no particular order (ok it's alphabetical/MM order):
Beholder: Classic monster implemented very well in the new edition, and seems like it could be very challenging.
Demon, Glabrezu: I've always liked this one for whatever reason, and it's 5e incnaration is probably one of the more powerful outsiders for its CR.
Dragon, any: I like how Dragon's are rendered in general in this edition. The lairs are a great addition (to other creatures too, but quantifying 'the dragon's lair' seems especially important to me), and I like how the spellcasting variant is designed.
Dryad: I like this as it doesn't try to overplay the creature and just focuses on it's core nature - a few tree-ish spells, and it's classic powers. The charm would make it scary in any larger encounter with other plant critters or fey, or it could serve as an interesting roleplay challenge. Also really eye-catching art (it's sexy without being porny).
Elementals, any: They're well designed on the whole, although I wish we had higher CR versions.
Helmed Horror: A cool lower-tier monster that I could see being pretty terrifying to PCs at that level.
Hydra: I really like how they did the Hydra's core mechanic in this edition. Better bring fire!
Lich: I like that they didn't skimp. 9th level spells, immunities, true sight, etc. In its lair, with its reaction spells (shield and counterspell) and countless other tricks, it definitely fits the status these guys are supposed to have in the lore.
Medusa: Gaze attack without punches pulled, including a nod to the mythology. Really cool picture, also.
Ooze, Gelatinous Cube: Yeah, I have a bias for the classics. But I like how they did its mechanics.
I could say great things about tons of the art, too, even beyond this list, although I feel like they really messed up the Invisible Stalker 
Neonflux wrote:
[quote="danyc"]
Elementals, any: They're well designed on the whole, although I wish we had higher CR versions.
I hope the elemental myrmidons are end up being CR 6-9 if they ever give them to us.
The Aberrations as a whole really got a great treatment. Everyone in the "holy trinity" had a great write-up.
Aboleth: This is the first time I've really liked the art for this creature. Previous editions made it look like a drunk dolphin doing cosplay. Now they look much more like a Lovecraftian horror.
Beholder: Yes! From the art on the cover, to the "dream origin" of the undead beholder to the lair and realm effects. Superb.
Mind Flayer: Solid threat and great mid-level masterminds.
Some classics also got fun entries.
Litch: Reclaimed their place as a force to be feared.
Lizardfolk: Excellent description of a culture that does not share the same moral values as us mammalian humanoids.
Gelatinous Cube: Super fun write up.
Metallic dragons: amazing art and really cool write-ups
Merrow: awesome art
Myconids: so weird!
Slaads: more amazing art and description
Arcanaloth: They're like the foxes from avatar the last airbender, but evil.
Gibbering Mouther: its The Thing.
Homunculus: This is why you become a wizard
Hydra: It's classic monster and the one in the book is really vicious.
Mummy: Where's Brenden Frasier?
Oni: It's scary and blue!
- Flumph- There is some good that came from the Far Realm after all!
- Kuo-toa- They believe in their made-up gods hard enough to bring them into existence and be granted spells by them! How great is that?
- Mind Flayer- They in no way manipulated the WotC team into writing the MM's disclaimer. Because they don't exist. This is why you don't need to worry about your tasty brain being eaten. Why would you even think about that? Let's just go back to doing human things and not thinking about the brain-eating abberations that could only eat our brains if they were real. They're not.
- Bullywug- The idea of a race of frog people acting like nobles reminds me of that toad in Flushed Away, and makes me giggle.
- Marid- Bulbous fish people dressed in fancy formalware just sounds funny. Especially since they like making up stories about how great they are.
- Mephit
- Pixie
- Scarecrow
- Flying Snake
- Faerie Dragon
Kraken and Aboleth. For both the art is the best I have seen. The aboleth actually looks alien and terrifying and the kraken has moved away from a giant squid and more like a kaiju from pacific rim. Both very cool.
I also really like the addition of lair actions for dragons, thjough I will use them as extra actions regardless of lair or not. The fit the magical element power I want for my dragons without having to go look up spells.