4. Boros

Here is a video just to prove how cool he is, nothing I say can do him justice.

Boros is the leader of the "Dark Matter Pirates", a group of alien invaders that come to Earth during the Alien Invaders Arc. He suffers from a similar crisis to Saitama, being that he is so strong that nothing thrills him anymore. Boros came to City-A in order to find a warrior of prophecy, a fighter so strong that he could even match up to himself.

He is by far and large the strongest character in the anime up to this point (other than Saitama of course). Not only is he the only character to survive a punch from Saitama, but he survived two of his normal punches, his "Consecutive Punches" attack, and finally fell to the "Killer Move Serious Series; Serious Punch". While Saitama didn't actually have to try to win this fight at all, that is still a lot more than what literally any other character can say about themselves.