Source: dccomics.com

5. Bizarro

You shut up, Bizarro is great.

Seriously, people love to hate on Bizarro, and I don't really get why. I guess it's because he's one of the few silly Silver Age villains who didn't get less silly, even as he was reimagined. But that's why Bizarro's so great. He's not gonna suddenly get all dark and gritty and start eating babies, or whatever. He's a silly, backward-talking monster, and Superman occasionally needs to deal with him.

To understand why Bizarro's a good villain, you need to get how villains usually work. A villain has to challenge some existing aspect of the hero. For instance, Doomsday was designed to challenge Superman's physical strength, and Brainiac challenged his intelligence. Bizarro challenged Superman's compassion, which was probably the most important part of his character. Superman's goodness was the most important part of him, and Bizarro tested that constantly.

He's not a problem to be solved through punching or thinking; he's a problem to be solved through understanding.