12. Doomsday
The ultimate generic force of destruction, Doomsday is not a nuanced, Shakespearean villain. He has no complex, nuanced objectives, despite what writers try to do. No, Doomsday is a force of sheer, unyielding power. He's the guy who can kill Superman. The Death of Superman is one of the most iconic stories in comic history, and it would be extremely awesome if, down the road, they can show him die. Of course, Doomsday's appeal in a JL film is that the League can't stop him. No matter how much they try, it's almost impossible to kill Doomsday. It would force the members together, and push them all to their absolute limits. In terms of a sheer fight, it would be incredible to witness. Perhaps too epic for film. That's how awesome that fight would be.