5. Doomsday
Though he does have slight changes to his history over time there is one consistency of Doomsday - He is an unstoppable, remorseless, and chaotic killing machine. Unable to comprehend feelings and emotions, Doomsday does not know fear or pain. In his most famous appearance, "The Death of Superman" Doomsday proves he is one of the mightiest beings in the DC comics universe. Bane was the first to break the Batman and Doomsday is known for "killing" Superman. Without giving away too much of the storyline it's obvious that Kal-El was able to survive and finally beat Doomsday. Doomsday would be a foe that both the Dark Knight and Superman could use as motivation to work together. The only drawback would be the fact that traditionally Doomsday only presents a physical threat, leaving all heavy dialogue and character development to the rest of the cast. Maybe Batman and Superman don't need a punching bag? What if they need someone that has the sadistic mind needed to pit them against each other?