Source: marvel.wikia.com

4. Maestro

So I tried to stay away from just populating this list with alternate dimension versions of Bruce Banner because it felt like cheating, but if there's one of those distinct enough to be a separate character, it's Maestro.

Maestro debuted in Hulk: Future Imperfect as a terrifying, super smart potential future version of The Hulk who had become smart and strong enough to kill off all of Earth's heroes.

He wound up in conflict with the regular Hulk, who beat him by warping him with Dr. Doom's time machine back to the Gamma Bomb detonation, where he was annihilated. Or so it seemed. He eventually re-constituted himself, and became a threat to the world at large.

There's also a different Alt-Earth maestro who was in charge of the Contest of Champions comic that I'm 99 percent sure was just a tie-in to the mobile app. The more you know.