I'm not big on superheroes but I've been watching the Marvel films and their actually really good. From what I know I'm going to have to say Iron Man. I always had fantasies about power armor suits when I was young; Iron Man is right up my alley. I think what I like most about superheroes is when they feel real. Superman very far from real, his super powers strike me as silly. Iron Man on the other hand just uses technology. Watching the Iron Man films the whole thing felt too down to earth to be a comic book film, the world actually felt very real (minus the origin story which I don't like). I'm kind of disappointed about the shared universe thing actually, with Thor and Iron Man existing in the same world, the feeling that that world is real is gone. Not that I don't like Thor, I do. I'm just not sure they're meant to be in the same movie. That said, I'm willing to accept it and am looking forward to The Avengers.
Captain America also surprised me. Even when I was a kid, I disliked the idea of "Captain America" thinking of the whole idea as stupid and forced. The few times I encountered him in cartoons I wished he'd go away. But then when I started learning about the upcoming movie I saw the whole thing started out during WW2 as a means of stirring up patriotism. The justification that the idea arose in such a dire time suddenly made me realize I like the idea. It arose for an actually reason; it isn't just forced patriotic tripe, it's very believable (not the super-solider serum, the results of it). I actually find myself liking Captain America, didn't expect that.