Interesting... not a single person has said the same thing up until now. I'm gonna go with ghasts, I think, though I'm also a fan of guardians, especially the Elder ones.
I like the enraged creepers... they remind me of the time i owned my own little army of creepers to take over EMC!! (Oh god I've said too much) um... *poof*
I'll have to go with bats... Why? Well, they're squeaky, annoying, serve no purpose whatsoever, generally get in the way, can't be egged, are hard to capture/remove... Nothing gives you more satisfaction than swatting one of those things into a leathery pulp.
Oooh, the best is when you shoot 'em down with a well-placed arrow. Nothing like a bat shish-kebab to warm the cockles of your heart.