9. Doomsday
The story of "The Death of Superman" may just be the most popular Superman comic of all time and we have Doomsday to thank for that. The only villain to ever kill Superman, Doomsday is a beast of pure power. Doomsday is a killing machine that will stop at nothing in order to destroy Superman. Created by a scientist named Bertron, Doomsday was born/created in prehistoric times in Krypton. He eventually turned on his creator and eventually set his eyes on The Man of Steel. There's no other way to describe Doomsday other than a mindless monster who is fueled by rage and hatred. Not only is he nearly invulnerable, but he has super strength, speed, regeneration abilities and has protruding bones that can pierce kryptonian skin. He also can't be killed the same way twice. He's been killed on several occasions but because of his genetic makeup, he's always able to come back. Doomsday is an absolute monster.
His Weakness? Superman's killed him once, as has Imperiex by using his energy beams. Brainwashing has been one way of defeating him (even though he's considered a mindless drone) and he's been hurt by Darkseid's omega beams. Other than these scare instances, Doomsday is a just a killing machine.