I'm thinking casting an unknown might be best, but here are my best bets from people currently working consistently in Hollywood:
Alex O'Loughlin - He's buff, he is convincing working as a cop and he's shown he can be funny at times on "Hawaii Five-O." Dye and grow his hair out a bit and I think he'd be great.
Karl Urban - He's a versatile actor who's well-versed in action. The problem would be I don't know if he can play the deadpan humor and I don't know if he really feels like a "superhero" kind of guy.
Considered people like Ray Stevenson (too old), Brandon Routh (too goofy), Chris Hemsworth (too young) and Wentworth Miller (too strange-looking), but it's a tough role to cast.