Enver Gjokaj as Superman
The beauty of the team up movie like Justice League and Avengers before it is you don't have to waste time tracking down the actor who can pull off both the hero and the alter ego. This allows great freedom in casting. My first choice is going to be Enver Gjokaj as Superman simply because the man has that look. That and he also has a great acting style. If you are scratching your head at his just go to Netflix and watch Dollhouse. Start for the Whedon, stay for the superb acting. Gjokaj isn't lacking in the muscle department either, so that doesn't hurt.
Oiliva Wilde as Wonder Woman
Wonder Woman is such a tough call. How do you live up to the legacy of Lynda Carter? My best guess is Olivia Wilde. She is bold enough and she isn't afraid to kick a little ass. Just look at Tron Legacy.
Chris Pine as Aquaman
Aquaman is going to be extremely tough to cast, simply because everyone seems to think he is absolutely useless. Anyone who thinks that though has not read the recent New 52 reboot of Aquaman or how he kicks ass in the Justice League. Pine can play dry wit very well and shows his ability to handle intense action scenes in Star Trek.
Neil Patrick Harris as The Flash
NPH is long over due to have a summer blockbuster while in the off season of a successful TV show, and what better way to do so than with a ensemble based hero flick? He is not afraid to cheese it up every once in a while and that is everything that epitomizes the Flash.
Vin Diesel as Martian Manhunter
This man is not afraid to let his nerd flag fly at all. Look at Riddick, a B-rate sci-fi franchise that he always ends up going back to visit. Regularly known as a comic book fan and closet nerd I am sure he would eat up the chance to be a part of the Justice League, especially as a character known mostly to the hardcore crowd. Slap some martian paint on him and have him talk in his deep brooding Riddick voice and you have a winner for the Manhunter.
Jim Sturgess as Green Lantern
While Green Lantern got his time on the silver screen it wasn't exactly a great film, so I wouldn't be surprised if for this iteration of the League another one of the five human Green Lanterns would be used. Sadly I don't see Simon Baz showing up as he is just too new, although I think that would be a great way for Warner Brothers to be bold and really show the Justice League as a world wide force to be reckoned with. Assuming they play it safe and go with the second human Green Lantern, Kyle Rayner, I would go with Jim Sturgess as the up and coming what-the-hell-am-I-doing-here hero digging deep to find his inner strength. If they go with Jon Stewart, Dwayne Johnson would be my choice.
Josh Lucas as Batman
This is the toughest casting call honestly. Batman is the DC Comics staple character (or Superman, depending on who you ask) but in order to make a Justice League movie work, you need to embrace the comic book version of Batman instead of the graphic novel version of Batman that Nolan introduced to the masses. You need someone who can handle being paranoid of every member of the League and extremely detail oriented. Plus he needs to be able to get into a shouting match with Superman and have the stones to back it up. Because we don't need a version of Bruce Wayne for the Justice League to work, Lucas is my choice for the caped crusader.