Garou

Race: Human
Age: 18
Height: 177 cm (5'9¾")
Abilities:
Superhuman Physical, Prowess, Supernatural Senses, Afterimage Creation, Accelerated Development, Indomitable Will, Technique Mimicry, Regeneration
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 villain, 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.
(Source: One-Punch Man Wiki)