Geryuganshoop is at number 6. Though killed in one punch by Saitama, his ability is extremely powerful. Most of the S-class heroes would have a hard time countering his gravity attacks. And his telepathy while inferior to Tatsumaki, is still extremely strong.
Fifth place is Bang. A powerful martial artist, he is capable of surviving Melzalgald’s direct blow with very little damage. His counter based fighting style puts him in a massive advantage against humanoid based opponents. Among the known S-rank only Tatsumaki would give him a hard time.
Thanks to his high speed regeneration and powerful strength Melzalgald is number 4. Though defeated at the end, he’s capable of fighting multiple S-class heroes at once. He also killed the Sky King in one blow, which is a dragon class monster like the Sea King. If the fight against the S-class heroes was one-on-one he’d win no doubt.
Levitating entire buildings all at once, calling meteors from the sky and deflecting an alien battleship’s main barrage back to itself. Her achievements speaks for itself, the loli no-pan telepath is extremely powerful. Her position as a rank 2 S-class is not for show. She could probably win a fight against the rest of the S-class heroes all at once.
2) Lord Boros
Boros is undoubtedly at number 2. He was the first opponent that withstood Saitama’s normal punch. He was also able to regenerate back after being pulverized by Saitama’s consecutive normal punches. He also literally punched Saitama to the moon. At his full power, he’s capable of moving at lightspeed, has an insane regeneration rate and can use a planet buster special attack. This makes Boros undeniably stronger than rest of the cast combined. (Minus Saitama of course.)
First place is obviously Saitama. His punch is capable of killing dragon class monsters and beyond in one hit. His tanking capabilities is top-notch, capable of being punched to the moon and back with no ill effects. But most of all, his serious series serious punch is capable of offsetting and canceling a planet buster attack. When was the last time you see a planet buster attack being defeated by a mere punch? And that was apparently him holding back. His power is limitless.
References: One Punch Man Wikia