Jumping on the monster band wagon here; I'm probably missing a few, since I don't actually have my Monster Manuals around to flip through.
1. Yak Folk. Shapes-shifting sorcerers who can summon genies and like to capture slaves? Sign me up. Motive, modus operandi, and a sense of style. Pity I never got to actually use one in a game -- I had plans, but the campaign ended.
2. Yuan-ti. Used them once, in the same campaign that was going to host Yak Folk. I'd dropped hints that there was this psionic Yuan-ti empire off in the distance, scheming schemes and causing trouble, but then the campaign ended, as previously mentioned. At any rate, I've got this thing for the darkest reaches of the jungle, and ancient temples built by degenerate civilizations, and Yuan-ti fit that bill nicely.
3. Ochre Jelly. I almost used these, once, in the session that became Gnome Town. That was originally going to be sort of a horror scenario, and I thought it'd be great if they started dropping from the ceiling and dissolving people. Someday.
4. Gelatinous Cube. Never used this one in a game, either. Noticing a pattern, yet? My D&D games in high school were never long on monsters, I guess. But gelatinous cubes are just cool. I don't know how you can't like a gelatinous cube. If I ever run 3rd edition again, I'm going to be sorely tempted to engage in some template foolishness -- you can give one an Int score high enough to take a class with the Fiendish Creature template, and then you're only a few levels of Monk away from a cube with a speed of +35 ft. and the ability to deflect arrows.
5. Naga. Mostly because of Nagathas, from the Monster Manual IV -- humanoids that spirit nagas have kidnapped, to twist into dark servants. (I always thought it'd be neat for that to happen to some NPCs the party knew.) But the whole guardians-of-ancient-magic thing rocks pretty hard, just on its own.
6. Werewolf. Mostly for the classic monster mojo; who doesn't like to play "guess the werewolf?" as peasants get slaughtered? I've also had some fun with them in campaigns past, using tribes of them as the "semi-hostile humanoids living uncomfortably close to town." They're a good mix of civilized and crazy monster, which makes them very versatile.
7. Golems. I've always loved robots and created beings in general, and I like the particular aesthetics that golems have. Unfortunately, I've never had a group get high enough level to really deal with a golem -- but next time I run D&D, I'm not going to let that stop me.
8. Blue Dragon. Why I didn't use these in the desert game I ran (the one that was going to have Yuan-ti and Yak Folk) I'll never know. Never got around to it, I guess. These are my dragon of choice, because I like the idea of dragons who think of themselves as lords of their domain, but are still more interested in dragon-y things -- eating sheep, collecting treasure -- than actual politics.
9. Storm Giant. Giants in general are a big list of creatures I never used because I never ran a high enough level game. At some point, I'm going to run a very giant-centered game, where most of the known world is ruled by different clans of giants. The players would be the members of the oppressed tiny folk small enough to fit into those weird catacombs that the giants don't know about.
10. Constrictor Snake, Giant. Um. Yes. Every campaign I've run for the past, oh, five years, all except the first two, have featured, at some point, a giant snake that tried to crush and eat the party. I don't quite know why, except that it's fun. Who doesn't like it when a giant snake tries to crush and eat the party?