Source: sciencefiction.com

9. Prometheus

Grant Morrison is very good. And he created Prometheus.

Real name and motivation unknown, Prometheus rigged up a helmet with a disk drive (ah the nineties) that would let him load skills into his helmet on the fly to counter the specific members of the Justice League.

He found the winner of a "be a Justice Leaguer for a day" contest, killed him, and replaced him, infiltrating the satellite and taking out pretty much the entire league single handedly. He would've won if not for Catwoman hanging around. He would come back later on in Cry for Justice where he caused the death of Arsenal's young daughter (it's not very good, don't read it) and then Green Arrow tracked him down and shot him in the head with an arrow.

He's also in JLA/Avengers, where he fights Captain America and Captain America schools his ass. It's pretty great.