Ultron
Thanks to the comic book masterminds John Buscema and Roy Thomas, who created this beast. Ultron first appeared in The Avengers #54 in 1968, was originally created by Hank Pym. However, the cinematic appearance in Avengers: The Age of Ultron shows the character as the brainchild of Tony Stark and Bruce Banner. Ultron possesses high cybernetic intelligence and is one of the strongest supervillains.
Thanos
Thanos first appeared in Iron Man #55 in 1973 and since then has shown to be the strongest supervillain of the Marvel world. He is so strong that he once killed all the superheroes individually and killed millions of people in just one go. What else do you need to prove he is the strongest villain in this list.