Who would win?
Saitama (Limitless) vs Cosmic Armour Superman
Saitama can punch through anything inside his world, but Cosmic Armor Superman isn't really inside it. He's Superman piloting the Thought Robot from Grant Morrison's Final Crisis: Superman Beyond, a being who exists outside the story and treats reality like a page he can edit. Saitama has no proven way to act on that level, so however hard he hits, he can't touch what he's fighting.
- Cosmic Armor Superman exists outside the story
- Saitama has no way to act on that level
- Raw strength doesn't bridge that gap
The hero of One Punch Man who beats everything with a single punch. "Limitless" is a hypothetical version: his strength keeps rising with no ceiling, taken to its end point.
Superman piloting the Thought Robot from Grant Morrison's Final Crisis: Superman Beyond, a giant machine kept in the Monitors' realm outside the multiverse to fight Mandrakk the Dark Monitor.
The full breakdown
For the stat fans: every check the engine ran, fighting in character and with no holding back (engine v1.8.26).