On the one hand, if his conceit of "one serious punch will eat any opponent" holds true across all power levels, the only way for someone to last against him would be if they could go intangible, like a Martian Manhunter sort of deal.
On the other hand, Anime Saitama has only been shown to have to serious punch a Saiyan-Saga Vegeta level bad guy. (Kill the earth with one big energy attack.). Considering Zarbon or Dedoria were capable of that, well... If we go web-comic Saitama, then after his fight with Tasumaki, I'd put him around at least Captain Ginyu. The problem with getting an even slightly accurate read on Saitama's power level is that he needs to struggle with something.
So, yeah. Squirrel Girl or something would be the only "fair" fight.
On the other hand, Anime Saitama has only been shown to have to serious punch a Saiyan-Saga Vegeta level bad guy. (Kill the earth with one big energy attack.). Considering Zarbon or Dedoria were capable of that, well... If we go web-comic Saitama, then after his fight with Tasumaki, I'd put him around at least Captain Ginyu. The problem with getting an even slightly accurate read on Saitama's power level is that he needs to struggle with something.
So, yeah. Squirrel Girl or something would be the only "fair" fight.