Ultron

Ultron is a robot created by Hank Pym, who became sentient and is the Avengers' biggest enemy. He will also be the main antagonist in the upcoming 2015 film The Avengers: Age of Ultron and will be voiced by actor, James Spader.

The Avengers: Earth's Mightiest Heroes
Ultron robots were the guards of the Big House, a shrunken prison. When a massive breakdown happens, the Big House recovers its normal size and the inmates escape destroying some Ultrons.

During Kang's invasion, Pym repurposes the Ultrons as an army, and adds the concept of violence on their brain circuitry. Of the Ultrons present in the final battle, only Ultron-5 remains. Ultron-5 also helps against Malekith the Accursed's Dark Elves.

Ultron-5 decides humanity is flawed and tries to destroy it. He first manipulates other supervillains like the Serpent Society and the Super-Apes to keep the Avengers busy, and then tries to destroy them manipulating their machines. Ultron-5 is defeated, but then returns as Ultron-6 and tries to get the nuclear codes from the Helicarrier. Pym defeats him by making him notice that, as he's a human creation, he's as flawed as humanity, and Ultron shuts himself down.

Ultron returns with his creation, Vision, who gets adamantium to create Ultron's new body. Ultron tries to replace the Avengers with robot duplicates, but Captain America defeats and supplants his own duplicate, and Vision betrays Ultron. Vision unlocks Ultron's head from his body so Thor can hammer it out using Mjolnir.