6) Tahmoh Penikett
Though he might not be as familiar a name as others on this list, Tahmoh Penikett would be a great choice for Superman as he has proved in his many supporting TV roles that he can play complex heroic characters.
You might think Penikett being Canadian would stand against him embodying one of the great American heroes, but the British Henry Cavill is playing Supes on the big screen, so there’s no reason why Penikett couldn’t take the role on TV. He also has big geek appeal from his roles in Battlestar Galactica and Dollhouse, so he would certainly fit into the Supergirl ethos.
Moreover, the actor has previously appeared in two other iterations of Superman – he played Jed Eubanks in Man of Steel and Wes Keenan on Smallville. Supergirl has cast several people with links to the Superman mythos before (Dean Cain, Helen Slater and Laura Vandervoort spring to mind), so this is just an extra tick next to Penikett’s name.