2. Metallo
Once again, the origins for John Corben/Metallo vary because of his long history, but usually after an accident Corben's life is saved by a group of scientists who transfer his brain to a robotic body and infuse Kyrptonite into his chest, the source of his life and power. In the modern reimagining of his origin, Corben is an army officer who volunteers in a program designed to take out Superman should he ever rise against Earth. The army origin has a lot of potential in the world Man of Steel established. The army is still unsure where Superman's allegiances lie and if they can truly trust him. Initiating the Metallo program is a sure way to focus on the distrust some in the military have while providing a villain who can be both a physical match and a weakness for Superman. As I said with Parasite, this also brings in the opportunity to show Superman using his intelligence to win in a fight rather than his strength. In addition, Metallo is basically Terminator with Kryptonite in his chest, making that a pretty cool visual to imagine.