Based purely on physical strength...
Superman - Superman's (depending on author/time period) is functionally infinite.
Kirby - Kirby can crack an Earth sized/density planet with one punch if he wants.
Goku - Goku's strength is demonstrated at mountain destroying levels with ease (shown when he punches people through mountains) though he may be capable of destroying a planet with his bare hands too.
Cyborg Raiden - He's shown to be able to lift giant mechs with ease (including using the arm of one to duel against it's other arm as if the thing weighed nothing...)
Though if we're looking at other powers...
Charles Xavier - One Punch Man forgets how to throw a punch.
Jean Grey (Phoenix) - Incinerates him in seconds.
Bobby "Ice Man" Drake - Freezes Saitama to death in seconds.
Martian Manhunter - Telepathy.
Death (various incarnations)
Lelouch - Could probably command Saitama to one punch himself or to never be able to attack again or something.
Alucard - (Especially if he has Schrodinger's power) because he can't be killed unless he releases all his trapped souls (and he has something like 3 million... And he always retains a few in reserve.)