Milo Ventimiglia, he's already got the hair.
Ryan could actually bring some depth to this character, or at least attempt to. I'd like to see a Supes with something a little more going on behind their eyes.
He already is America's number one guy right now, whey not throw in some super powers. Jon Hamm, forever.
Doomsday was just a bad tease, Sam Witwer has the troubled brow that could play the "alien on Earth alone and sad" movie all the directors in Hollywood are so desperate to make. He could be Dark Superman.
If they ever wrote a script interesting enough to entice Leo, you can probably already bet it's going to be a solid Superman.