Hiro has his moments, and its always fun telling people that my favorite superhero is action bastard (and seeing how they react, when most of the time they have no idea who I'm referring to) but my favorite would have to be the Happiness Bunny.
"No Longer am I the happiness bunny, forever I am the bunny of misery and dispair. Please kill me." - Happiness Bunny
My favorite episode, obviously, would be The Happiness Bunny's Revenge. Also, the fever episode ("Lets show this flame-broiled b***h who's boss!").