Ok, I admit it. I'm Chaotic Difficult. But really, it's hard for me to pick just one. I'd even be hard pressed to pick one type (humanoids, animals, oozes, vermin, fey, magical beasts, dragons, outsiders...). I guess I don't have a favorite, because my favorite thing to do is create unique monsters and present players with a diversity of challenges. In fact, one thing I really dislike about 3rd edition is how fast the characters progress in levels. It annoys me whether I'm a DM or a player. You don't get enough oppurtunity to use the abilities you have against a wide variaty of challenges before you are gaining new abilities.
But, ok, if I can't pick alot of favorites, I'll stick with Dark Creepers.
And Dark Stalkers. (But that's just one choice, really. No really.)
But seriously, what DM really wants to use just one monster - no matter how cool. I mean I don't know how the guy that designed the A region of the world's largest dungeon limited himself to Darkmantles and Stirges, because I could never be that monotonous; but I do know that he never should have limited himself to just Darkmantles and Stirges no matter how cool they are.