14. Starro
The enemy that united the Justice League in the first place, this is one strange starfish alien with mind-control powers. He's less a straight forward villain, and more an eldritch abomination. In the right story, he's an unfathomable threat, able to assume control of a host, and use to further his own objectives. With this sort of power, he could, say, take control of Superman, and use him to take over the world. Starro is the type of villain you assemble a team to fight. It's almost impossible to fight him alone, since you could easily be taken over. Many people watching a JLA film might question why Superman would need Batman or Wonder Woman's help to stop a threat. After all, he's an all-powerful hero. He can stop anything. Well, if Superman is the threat, yeah, he's going to need a little help. Again, this could also justify a lot of hero vs hero fights, which would be really cool on screen.