Henry Cavill as Superman. Or, if it's not to be in the same continuity as MOS, bring back Brandon Routh.
Armie Hammer as Batman (unless the Lone Ranger's flopping has nixed it, I still think he has a good chance of playing the role in the next reboot).
Ryan Gosling as The Flash. 10 years ago, I wanted Owen Wilson, the slowest-talking man in Hollywood, to play the fastest man alive but I now think Gosling's slightly spacy persona would be a nice counterpoint to his superpower.
Cody Smoulders would be a good Wonder Woman - I don't see her involvement in the MCU being a particular problem.
Martian Manhunter is going to be under CGI or latex anyway, so they need to have a distinctive voice at least. I'd probably want a black actor but not someone with an 'urban' American accent; I'm thinking of someone like Michael Dorn was in TNG, deep voice, but not 'street'. Maybe Idris Elba, even if he's a tad-over exposed to fanboy wish-casting.
Aquaman I can't really think of, truth be told.
Green Lantern - Chris Pine. Maybe a little close to his Captain Kirk, but hey it's fantasy, so...