Darth Vader in all the Star Wars games. Also on the normal Xbox, those Darth _____ characters. They are all so very evil. But Darth Vader is number one since he is known more. Also he is quite difficult to defeat in most games.
The Joker obviously. He's so funny, and absolutely insane in Arkham Asylum. Oh and since Harley Quinn is one of his lackies, that gives him another thumbs up for me.
Easily the Creepers. Those ****s will piss you off until you can kill enough spiders and get lucky enough to get 3 string and be able to make a bow to kill those mothers from a distance. until then, they will DESTROY YOUR HOUSE!!
I'm going with the Pacman ghosts. They will make you so angry when they just chase you around until they eventually kill you. But it's fun when you eat them all.