Darkrai definately. Mostly because I can watch and sneak up on hot babes and give people sadistic nightmares when I'm feeling evil. >D
Unfortunately, that strategy goes down the tubes when you don't have a bottom half... XD
I would like to be like Unown, Live in that little dimension they showed in that episode i forget the name of. Would be quite crowded.
Also i would be electrode or voltorb and just go on a shock spree for the hell of it.