Finding a good spot to read that specific showdown is tricky because most of the major archives don’t really sort by 'best fight scene.' On Archive of Our Own, you’ll have the most luck—tagging is meticulous, so you can search the 'Saitama/Boros' character relationship tag (even though it’s platonic/rivalry here) and then filter for 'Action' or 'Canon-Typical Violence.' I’ve read a few that extend their battle on the alien ship or explore a 'what if Boros survived' scenario, and the writers really go all out describing the sheer force of the punches. Sometimes you get a slow-burn from Boros's perspective, which adds a weird pathos to the whole thing.
That said, FanFiction.net has a higher volume of older, straightforward power-wank fics where the fight is just a blow-by-blow retelling with more gore. The quality varies wildly, but if you want pure, unadulterated spectacle without much character depth, that’s your old-school haunt. I’d cross-check favorites and reviews there to find the ones people still talk about. Wattpad can be surprisingly good for this too, but you need to sift through a lot of reader-insert stuff first; their algorithm sometimes surfaces hidden fight-focused gems if you persist.