Discussing which main character is the most powerful is like a playground fight between kids where everybody gets the everything-proof shield. Yet, in spite of the futility of such arguments, there’s a lot of interest in content that compares the strengths of various characters and decides which among them would win. I mean, just look at the explosive success of Death Battle.
ONE’s star franchises, that being One Punch Man and Mob Psycho 100, are parodies on this exact aspect of nerd culture, which expose the inherent lack of value in these discussions. ONE’s answer to “what character is the most powerful” is to just invent characters that are, by their very nature, always the strongest. Saitama will always defeat any foe of any power level in a single punch and Mob is just the most powerful psychic in existence, Period. There’s no comparing to them and they always win.
It’s an effective way shut down the discussion. Goku can’t beat One Punch Man, he’ll just die in a single punch. Neither can Superman or Spiderman, nor any other hero or villain. If they have some magical reason that prevents them from dying in one punch, then they’ll just die on the next hit, or get beaten so thoroughly each and every time that they’ll wish they were dead. One Punch Man states this a lot more bluntly than Mob Psycho 100, but both get the point across.
At the same time, I don’t feel like either story was intended to shut down lighthearted, fun discussions about how cool our favorite heroes are. The joke is directed at those who take these conversations too seriously, and are genuinely determined to prove that their favorites could totally beat up Batman or whatever other protagonist their current conversation partner prefers.
Being able to put these characters against Saitama or Mob brings to light the pointlessness of these hypothetical cross-over scenarios. Does it matter if Link could beat up Dante or if Kiss Shot could take on Taiga Aisaka? Not really, does it? What matters is how strong these characters are within the context of their own, fictional words. Kirito doesn’t need to be concerned with how he compares to Eren Yeager, so long as he is strong enough to resolve his latest isekai adventures while Eren is waging his war in a different reality altogether.
The challenge isn’t to come up with a character who could beat One Punch Man, it’s to focus on your own writing and create characters that make sense within the context of your own world.