Who else? Like the rest of the villains listed here, Ultron has yet to conquer Earth's Mightiest Heroes, but he's brought them to their knees more often than anyone. Whether it's destroying entire nations or very nearly conquering Kree space, the near indestructible Ultron has returned again and again to battle The Avengers and bring down the man who created him, Hank Pym (which he recently succeeded in doing). One of Ultron's greatest victories came when he finally took over the Earth and wiped out all but a few of The Avengers. A time travelling Wolverine managed to put things right, but this was the closest Ultron came to breaking his enemies for good. However, he won't rest until he succeeds in destroying them, and so long as there is an Avengers, you know there will be an Ultron there to take them on again and again!
