Well, it would never happen, but I've always liked John Milius. He directed the original Conan The Barbarian, and the original Red Dawn (an ensemble cast, like JLA would be), he also wrote the Apocalypse Now screenplay. He's a right-wing guy though, so Hollywood's given him the cold shoulder for decades; but he'd be my dream choice.
I think John Favreau would be a good contemporary choice as far as the Justice League movie goes, he and Sam Raimi basically brought comic book movies back from their 1990's depths with Iron Man and Spiderman, respectively. As far as Raimi, I like him, but I think he's a little-bit unpredictable for a JLA film. Spiderman 1 & 2 were good, but 3 stunk on ice.