6. Brainiac
If Hal 9000 had superpowers, this would be him. It is a commonly accepted fact that Brainiac is a great Superman villain, but people overlook that he can--and has--been a threat to the JLA. His powers can threaten the stability of the entire world, and what better way to gather information than studying the various heroes of earth? The great element would be the personal connection between him and Superman. As in the comics, Superman and Braniac have amazing chemistry and an incredible relationship. Brainiac can push Superman to his limits, and make him question his rigid code: maybe this one time, he can kill the son of a bitch. Not to mention that, with such a huge villain, there is the potential for Brainiac to hurt the rest of the JLA. Maybe he could conquer Paradise Island or Atlantis. Maybe he'd hack Bruce Wayne's computers, and learn he's Batman. The possibilities are endless!