Source: hollywoodreporter.com

3. The Hulk/Maestro

The Incredible Hulk, Hulk: Future Imperfect - Marvel Comics

The Hulk is always fun. He's also kind of dumb. But what would happen if he were smart? And evil?

Well, you'd have the premise behind the classic Hulk story titled Future Imperfect, in which Hulk was flung into an alternate future during which Banner and the Hulk merged to become Maestro, the ultimate Hulk. Maestro eliminated all the other super people on Earth and took control of the planet.

The writers liked the character, even after Hulk defeated him, so he continued to show up. This happened largely in works by Maestro creator Peter David, including Captain Marvel, 2015’s Secret Wars miniseries Future Imperfect, and the resurrection of Spider-Man 2099 from the 2010s.

Maestro was also spotlighted in the Marvel Now Contest of Champions series, which I really need to read because it's supposed to be awesome. But Maestro is a pretty solid heel turn for a hero who always took time to ride that line.