Jack of Hearts.
I just love this character, and I really like some of the recent changes made to him.
I think what appeals to me about him is that he's a bundle of angst. He's hot-tempered and frustrated by his seemingly uncontrollable powers.
He's stuck in an containment suit. He's found out that much of his life is a lie (i.e. he's only half-human). He's considered a major threat by the authorities (S.H.I.E.L.D.).
While he certainly has been labeled a hero, I don't really think so. Instead he's one of those types who just end up having to chose sides. He doesn't go out on patrol. He doesn't have any desires of revenge against criminals. He certainly doesn't have any strict code of ethics (which adds to the loose cannon fears S.H.I.E.L.D. always had about him).
Add on top of that his recent developments: The maddening issue that his powers can only be checked by being sealed away in a vault for long periods of time and his growing anger levels.
 ... Now that's an interesting character.
I know he's dead right now, but man, I've pined for a Jack of Hearts on-going series (or second mini) for years now.
Bill Mantlo did a GREAT job developing this character. I really miss Bill.
(FYI — Bill has been in a coma/iincapacitated for years now. He got into a very severe accident while rollerblading in the early 1990s.)