Saitama would either destroy Superman, or it would be a tie. Saitama has unlimited power, same as Superman. In the final episode of One Punch Man, Saitamas greatest rival yet, Lord Boros, says when they first meet "This Saitama has unlimited power. I can feel the power emanating from his body." And at the end of the battle when lord boros is about to die, he says that Saitama still had a ton of energy left, after taking a huge beating. So huge he went to the moon! And he didn't feel any pain. Saitama still has a lot more power to gain, but is it enough to kill Superman? I mean, he kills almost anything with one punch.