2. Iron Man
Given the contents of the recently released teaser trailer for Avengers: Age of Ultron, Tony Stark seems the most likely origin for Ultron, as said trailer appears to show a suit of Iron Man armour changing form to become Ultron. While this would give Tony Stark who appeared to retire at the climax of Iron Man 3 a valid reason to return to his superhero role, given that the emergence of the villain would be his fault and he would feel a responsibility to stop it, it could also give Ultron a connection to, and therefore potentially control over, any Iron Man armours that Stark has built come the time of this movie. In one comic, the Ultron program interfaces with Iron Man's armour, which Iron Man had integrated with his biology. This allows Ultron to transform Iron Man in to a new version of himself. If Stark does indeed inadvertently create Ultron, he will already have enough to feel guilty for, without Ultron actually using him to fight his own supposed team-mates - but it really could happen.