If you leave out non-combatants, NPCs (humanoids), and unique creatures, and count variations of monsters as one monster (skeleton, skeleton warrior, skeleton champion, etc as 1 type, same with Draugr, Restless Draugr, etc), the breakdown is as follows:
Daggerfall: 36 hostile creature types (giant rat, giant bat, grizzly bear, sabertooth tiger, spider, giant scorpion, slaughterfish, centaur, dragonling, dreugh, gargoyle, giant, harpy, imp, lamia, nymph, spriggan, wereboar, werewolf, air atronach, fire atronach, flesh atronach, ice atronach, iron atronach, ghost, lich, mummy, skeleton warrior, vampire, wraith, zombie, daedra lord, daedra seducer, fire daedra, frost daedra, daedroth (lesser daedra)) (source: http://uesp.net/wiki/Daggerfall:Bestiary)
Morrowind: 39 hostile creature types (ascended sleeper, ash ghoul, ash slave, ash vampire, ash zombie, corprus stalker, lame corprus, alit, cliff racer, dreugh, guar, kagouti, mudcrab, netch, nix-hound, rat, shalk, slaughterfish, flame atronach, frost atronach, storm atronach, clannfear, daedroth, dremora, golden saint, hunger, ogrim, scamp, winged twilight, centurion sphere, centurion spider, steam centurion, kwama forager, kwama warrior, scrib, ancestor ghost, bonelord, bonewalker, Dwarven spectre, skeleton) (source: http://uesp.net/wiki/Morrowind:Creatures)
Oblivion: 28 hostile creature types (wolf, boar, mountain lion, rat, mudcrab, slaughterfish, imp, troll, will-o-wisp, spriggan, minotaur, land dreugh, ogre, goblin, skeleton, zombie, ghost, lich, scamp, clannfear, flame atronach, frost atronach, storm atronach, daedroth, spider daedra, xivali, dremora) (source: http://uesp.net/wiki/Oblivion:Creatures)
Skyrim: 32 hostile creature types (bears, horkers, mammoths, mudcrabs, sabre cat, skeevers, slaughterfish, wolves, dremora, flame atronach, frost atronach, storm atronach, Dwarven spheres, Dwarven spider, Dwarven centurions, chaurus, dragons, Falmer, frostbite spider, giant, hagravens, ice wraiths, magical anomaly, spriggans, trolls, werewolves, wisps, dragon priests, draugr, ghosts, skeletons, zombies) (source: http://uesp.net/wiki/Skyrim:Creatures)
These lists do not include creatures that are hostile only if attacked. (Some creatures will attack only if the PC gets too close, like horkers. They are included because you don't have to actually attack them for them to aggro you).
I started a thread a short while ago complaining about the lack of diversity, but I hereby eat my words!

Edit: @Arzalar - yeah it takes a long time to make the lists, but its easier to get the point across!