8. Onslaught
Onslaught is a combination of X-Men characters Charles Xavier and Magneto. He's a being of energy that's made up of the characters two's psyches. His abilities include all of Professor X's as well as Magneto's. He can also transport his psychic energy into other beings, temporarily taking over the minds of his enemies. He's wreaked havoc on both the X-Men as well as The Avengers, which are quite possibly two of the three most powerful factions in superhero history (with The Justice League reigning supreme). Whenever he's involved in a storyline, Onslaught has caused mass destruction in the Marvel Universe. It took an army to defeat him.
His Weakness? He was once punched by The Hulk so hard that it temporarily separated The Hulk from his alter ego Bruce Banner. The punch momentarily broke Onslaught's psychic wall and allowed for the The X-Men and Avengers to simultaneously entire his mind and defeat him.