7. Ultron
A killer robot need I say more? Created by genius and superhero Hank Pym aka Ant Man, Ultron's artificial intelligence was an imprint of Pym's mind which means he started life already a superior intellect. Except Ultron kept learning, his mind expanding beyond the bounds of measurable intelligence, and eventually Ultron rebelled against Pym, becoming one of the toughest opponents the Avengers ever faced. Besides his great robotic appearance and extraordinary mind, he has an impressive array of powers ranging from mind control to concussive energy blasts as well as flight, super strength, speed, stamina, etc. His outer armour is also made from the fictional metal adamantium, making him all but invincible from most attacks. The character recently featured as the force behind the end of the world in Age of Ultron, a divisive Marvel Event comic that in fact featured Ultron only in the final issue. However Ultron was able to take over the world with his army of Ultron drones (clones of himself) sent from the future to the past, making him more than a match for all of the Marvel Universe's heroes - at least in that timeline. Though there isn't much more to Ultron than simply the villainous killer robot angle, sometimes a great super-villain is a classic archetype done really well and that's exactly what Ultron is. No wonder Joss Whedon picked him out to be the bad guy in The Avengers sequel.