23. Ultron

One of the biggest regrets in Hank Pym's life -- and the man, sadly, has several notable ones to choose from -- Ultron exists because of Pym's experiments with artificial life. The robot Pym first created could initially be called a success, since it did indeed achieve intelligent and sentience -- to a very dangerous degree, causing it to strike out at its creator and set out to become one of the Avengers greatest threats.

From his early days posing as the Crimson Cowl and leading one of the many incarnations of the Masters of Evil, Ultron has proved to be cruel, clever and resourceful. Constantly upgrading himself, his frequent name changes (Ultron 5, Ultron 6, etc.) predicted the modern era 'Version 2.0' idea of the ever-improving and streamlined piece of technology, here represented in the scariest way possible. The early upgrade that turned his entire metal body into adamantium only increased Ultron's strength and made it more difficult to stop him.

The Avengers have found that they can only temporarily stop Ultron. Able to exist within computers and other technology, Ultron always comes back; more difficult to defeat than the last time. His lack of humanity and disrespect for life makes him all the more intimidating -- Ultron is the kind of foe who can rarely be reasoned with and trying to find his sympathetic or compassionate side is a fool's errand (Ultron 12 aside).

Ultron's cold view of humans led to one of his most brutal and despicable acts, as he slaughtered an entire Eastern European country, Slorenia, showing no mercy and certainly no regret over his actions. More recently, Ultron's threat has become a cosmic one, as he took to the stars and led the Phalanx quest for conquest.