Garou

He was the best disciple of Bang's dojo, until he went on a rampage against the other students and disabled many of them for life. He was expelled after getting a beating by Bang. He eventually becomes the strongest villian, ranked as a Dragon + level threat (considered around the same strength as Boros). Using his 'God-Slayer Fist', he easily defeats Golden Sperm and most of the remaining S-Class heroes. In an epic fight with Saitama, the man is ultimately defeated but manages to escape before the heroes are able to get rid of him for good.