4. Crime Syndicate of America
This is a little curious. The Crime Syndicate is basically the JLA's evil twins from another dimension. They are everything the JLA could be if they used their powers for power and domination. Aside from the fascinating possibilities of the team fighting their doubles--and being forced to see themselves through a glass darkly--the more interesting aspect is how perfectly matched the teams would be in terms of power. Neither side would have an edge. They'd both be at each other's throats the whole while, neck and neck! Of course, the other interesting aspect would be Lex Luthor's alternate universe self: a noble hero, the last man in a planet run by terror. Even in the comics, the alternate Luthor and Superman had great chemistry. Superman is biased due to Lex's behavior in his own world, while Lex is thankful for the existence of heroes who could aid him in his crusade. Any story with these guys in it would be fascinating. Though, speaking of Lex...