Chris Hemsworth- I think he would have made a really really really good Poseidon in The Lightning Thief. He has about the right eye and hair color plus anyone who's seen Thor knows he can pull off powerful immortal being quite well.
Robert Downey Jr <----- I love Robert Downey Jr. He's so attractive and awesome actor! He made iron man cool. I was not a comic book person until i started watching iron man. His swagger is crazy cool but still ridiculously intelligent.