Honestly I feel like boros should be the strongest, outside of at least a further evolving monster garou
although this wouldn't really matter here since you can't scale from a casual saitama, Boros is really the only enemy besides Garou who was able to be relative with what casual saitama threw at him, and seeing what the sp^2 did, it's insanely impessive that he was able to temporarily survive a (somewhat weaker) serious punch
also if the stuff about him being the strongest in his species is true then I mean, he's probably at least boris level, who scales to blast being like far far lower into 4-A cause of square cube law serious punch^2.
Like sure, Saitama would have never lost no matter how it played out, but boros vs saitama is less like human vs single celled organism, and more like bodybuiler vs human child