2. Robert Downey, Jr.
Downey Jr. has kind of appeared in The Simpsons before - he was in a shootout scene with the police in the episode "Beyond Blunderdome" (in which Marge thinks he's shooting a movie scene, but Bart mentions that there aren't actually any cameras present) - but he didn't lend his voice to the role or appear for more than a couple of seconds. His wit and charisma is second to none and it would be fantastic to see him in Springfield. I imagine the stark (no Iron Man pun intended) contrast between the charming, dashing actor and the tactless, slobbish Homer would make for some very humorous interactions. Since revamping his career with the phenomenally successful Iron Man movies, Downey Jr. has become one of the most sought after men in Hollywood and his potential Simpsons appearance would be a massive audience attraction.