Source: dc.wikia.com

3. Metallo

John Corben was a soldier under the command of General Sam Lane. Motivated at least partially by an unrequited love for the General's daughter, Lois, Corben underwent dangerous and experimental procedures to fight Superman.

Unfortunately for him though, he endured a horrible accident and emerged more machine now than man.

With a kryptonite power source for a heart and a mechanical exo/endoskeleton depending on the version of the character, Metallo goes to work trying to kill Superman using his enhanced strength and Kryptonite heart.

Metallo would be excellent for a film appearance, and not just for his creepy visual design. Having a villain with a connection to Lois would give her a better connection to the plot than she's typically given in Superman movies, which would be a good thing. Come on, who doesn't want to see Superman punch the kryptonite-hearted terminator? Because I DO. I do so, so very, very much.