17. T. O. Morrow
A mad scientist responsible for the creation of Red Tornado and numerous other androids, the appeal of this guy would be less of the threat he'd pose himself, but rather the threat his creations would pose. Elemental machines thrown at the Justice League, the robots would be tragic characters, aspiring to be human, despite this being a complete impossibility. Mad scientists have been surprisingly absent from a lot of superhero movies. You have Doc Ock, sure, but most of the scientists have been on the side of the angels. A good villain does not need to be front and center in the story to have a huge impact. If his machines outlive their creator, then the audience can be treated to absolute chaos. It would be really cool to see the Justice League handle that mess.