5. Robert Downey, Jr.
If you were to ask people who the most famous current superhero actor is right now, I bet that people would either say Christian Bale or Robert Downey, Jr. While I will say that Bale and his smoker's Batman voice would be amusing to see on a network sitcom, I find that Downey has slightly more recognizability than Bale does on the superhero front, and thus would work better for a cameo. The thing is, his appearance should probably be just that: a cameo. Don't get me wrong, Downey is awesome. He happens to be one of my favorite actors. Still, with all of the potential he brings to an episode's story, I believe humor-wise that his appearance would work better in only one scene, similar to what they've done with celebrities like Charlie Sheen and Howie Mandel. Maybe one of the guys bumps into him on the streets of Pasadena or at a comic-convention, and they apologize without actually looking at his face, therefore missing the chance to talk to Tony Stark himself. Either way, I expect Downey to deliver one of his deadpan sarcastic replies. It's one of his many performance traits I've grown to love.