Superman -- someone besides Henry Caville. Very stiff and bland in the role. Maybe David Giuntoli.
Batman -- I think they made a good call with Affleck.
Aquaman -- Mike Vogel, who plays Barbie on Under the Dome. If they don't get him for this role, then they should definitely get him to play Iron Fist in a Marvel flick.
Green Lantern -- Matt Bomer, from the show White Collar. He's got the look, plus he's a solid actor. He also has a knack for playing the underdog, which GL typically is.
Martian Manhunter -- my personal favorite DC character. For a guy who could beat the snot out of the entire Justice League, Superman included, he doesn't get near enough love. I think they'd need someone who can be intimidating but who also has a certain maturity to them. Call me crazy, but I like Jon Bernthal (Shane from The Walking Dead) for this one.
Lex Luthor -- Dean Norris, from Breaking Bad and Under the Dome. Great character actor and plays a really good heavy.
Wonder Woman -- Jaimie Alexander. She could play Diana in between stints playing Sif in the Thor flicks.
The Flash -- Ryan Kwanten (True Blood). The Flash is a light-hearted character and Kwanten has comedic ability. And he's built like a runner, tall and lean, so he wouldn't have to do like most CBM actors and work on his physique.