We already asked how do you find a villain for someone as powerful as Superman? The solution was to create someone even more insanely powerful to oppose him. In this case, it was Doomsday, who was strong enough to kill the Man of Steel.
Doomsday is a primitive Kryptonian tortured by a scientist over time. He was genetically altered and then put on a hostile planet where he would die and then be cloned again with the memory of that death… and a natural resistance to what had killed him last time that evolved without the knowledge of the scientist. Doomsday became a creature of out of control evolution, who became an unstoppable force. How come mad scientists never think of this?
Doomsday lands on Earth finding the Justice League at a time when the team was full of losers. Basically Superman was trying to defend Earth with the gang from Scooby Doo. (Seriously? Bloodwynd?) As we know, it doesn’t go well.
Doomsday is, of course, absurdly strong and resistant to damage. He heals immediately from damage or evolves new powers as the writer thinks of them. He is also quite a bit faster than you would think.
Actually, I guess he’s already black and blue.
Doomsday also met the Green Lantern Corps. He killed hundreds and wounded thousands, mostly out of anger over their lousy movie. He confronts a Guardian and adjusts to his attack while while he’s fighting him.
Doomsday was a mindless destructive force that is controversial among comic book fans, but as he is written, he is unstoppable.