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In no particular order;


 1. Mimic
 2. Green Slime
 3. Jackalwere
 4. Wererat
 5. Rust Monster
 6. Son of Kyuss
 7. Succubus
 8. Carrion Crawler
 9. Doppleganger
10. Medusa            

In increasing order:

1. Bugs - Giant ants, kruthik, ankheg, remorhas, hook horror, cave fisher, Thri-kreen... Fighting giant bugs underground, and they even have mages? Priceless.
2. Goblinoids (bugbear, goblin, hobgoblin) & Orcs- Who doesn't like some hack&slash? Interesting characters, with many possibilities (goblin shaman, hobgoblin alchemist, bugbear trapper, orc warlord). I really like to add the "Waaagh!" element to them, they are attacking a town, pillaging a village or looting the neighbourhood. Easy adventure concept, many variations, a lot of fun.
3. Yuan-ti, Naga, Lizardmen & Medusa - Same as above, but with the possibility of intrigue & power schemes. I really hate to use doppelgangers/shapechangers, but I like to use illusionists. Yuan-ti are usually good illusionists in my world.
4. Sahuagin and Kua-Toa - Same as goblinoids but with an aquatic theme? Letting the players ride dolphins, with somekind of underwater breathing device, fighting the "old ones" and defeating huge krakens on sea? Add some pirates to the mix and you got yourself a water themed campaign.
5. Minotaur - Same as orcs, but more powerfull & more intelligent? Everyone knows them, you don't have to go back to the LotR lore to explain their existance, and they can be a lot of fun.
6. Frost giants (or giants in general) - same as orcs and minotaurs, but they provide an even higher level of challenge. Throw in some "slaves" like otyugh hous cleaners, or salamander servants, or wyvern pets, what have you and they can provide a lot of fun.
7. Humanoids - humans, elves, dwarves drow, goliath, warforged, thri-kreen etc. It is fun to fight monsters, but after some hack&slash it is imho a good idea to return to the city and do some investigating, some plotting, some sheming, some thinking. You can introduce a lot of fantasy elements, like goliaths who eat rock instead of normal food, warforged who can change their limbs and other bodyparts everyday, dragonborns who have large wings and can fly. Drows just in general can provide enough ideas to fill out a lvl1-30 campaign.
8. Demons - Actually I find to "roleplay" proper demon villains hard, but each time I try I really enjoy myself. ^^ Powerfull, but still have brains? Endless numbers, with limitless mutations? Elemental elements, or chaotic properties? Demons from "history" or demons from modern fiction, the variations and possibilities are limitless. And they provide very good Archvillains.
9. Devils - Your adventurers have been fighting undead from lvl1? Finally managed to kill the Arch-Lich who was behind all of this nonsense? But do they know who was behind the Arch-Lich? Devils! Devils betray each other, humans, undead, their servants,... anyone. They are variable like Demons, with the added extra that you can make them look like angels (or whatever) and they still feal like devils. With a large number of low-, mid- and high level devils, they can provide a challenge for more then just one campaign. Make them the ultimate brain behind all of the trouble the players had to face and you got yourself the ultimate arch-villain.
10. and my favourite, the undead. Killing zombies, hacking ghouls, banishing ghosts, defeating liches, wraights, vampires, wights, spectres, mummies, bone golems, shadows, skeletons and dracoliches just never gets old.
+1 Fey creatures like dryads, treants, pixies, are cool, but I usually never use them. They are cool nonetheless.
+2 Wererats (skavens I mean), are really great characters, with a lot of flavor.


I also would like to add my 10 least favourite, in no particular order:
Scarecrows: You are joking right?
Shapeshifters/changelings/mimics: They kill my games. Interesting concept, interesting idea, but the meta game involved just kills my games.
Beholders: a complicated and interesting abomination, so what?
Dragons: they are hoarding treasure and attacking the countryside, do they do anything else?
Kenkus: Birdmen, who can't fly? Seriously?
Mindflayer: I don't get them. I really don't understand this race. The problem is probably with me.
Kobolds: Kobolds are little man, hiding in the forest, making mischief, not some kind of gargoyle looking dragonlings... Who came up with this name? Would they be called drakelings or what have you, they would be fine for me.
Slaad & Bullywug: Lizardmen are fine, but I can't really imagine Slaads and Bullywugs.
Xivort & xvarts: Little blue man? You are kidding me?
Colossus, Sphinx, Cyclops: So colossi are giants, with a twist, cyclopses are giants with a twist, sphinxes are ..., what are they actually? I understand that more then 50% of the monsters are from the old greek mythology but these three aren't unique enough to justify their existence imho.
+1
Primordials: I really don't understand the "need" for them. What are they trying to be (other then epic lvl challenge)? Elementals but twisted in a new way?

Wow, I was just thinking of posting a thread with this very (exact...weird) topic; and this is not about general fantasy monsters, but iconic D&D Monsters, so, yeah:

-Aboleth

-Beholder

-Carrion Crawler

-Ettin

-Githyanki

-Lich

-Mind Flayer

-Nightmare 

-Owlbear 

-Umberhulk


...there are others...