(Note to all researchers working to create A.I.: Just STOP. It never ends well for humanity.)
Fighting the Avengers numerous times since his debut, Ultron is one of the most truly terrifying opponents for Eartth's Mightiest Heroes because no matter what they do, he KEEPS COMING BACK. No matter how many times they destroy him, he rebuilds himself, and comes at them again. And again, And again. And that's why he's so scary. Because the Avengers can never keep him down for good. And one day, sooner or later, ULTRON IS GOING TO WIN.