Saitama is filled with many, many fan boys that believe he can take on anyone, however I'm going to show just how powerful Saitama is while putting him up against someone I think would be an interesting opponent which is evil Ryu, so let's get started

Attack potency

Evil Ryu more than likely has an attack potency of large star level, he was able to take Asura in his 6 arms form and as I'm sure we all know Asura is no push over, he's also near the level of Akuma who is comparable to some of the most powerful Tekken characters.

Meanwhile Saitama is probably just star level, he is only really about the same level as Boros and I don't think we could try and say he's 100 times more powerful or anything like that

This specifically states that Saitama was "sort of clobbered" meaning Boros was correct when he said that Saitama took damage

So really I can't see Saitama being too much higher, possibly large star level however I kind of doubt it.

Speed

Ryu is definitely faster than light, he can keep up with Asura and he can keep up with Akuma

There really is no doubt in my opinion of Ryu being faster than light.

Meanwhile Saitama.... Probably isn't, it's hard to say if he's even light speed, however let's just say he is from that feat of jumping from the moon to the earth in a few seconds and that he was just not trying and that's why it took him a bit of time.

Conclusion

OK I need to be honest, even if these two had the exact same stats or even if Saitama had slightly better stats evil Ryu would still win, he is more ruthless, he has more abilities and he is a better fighter, Saitama doesn't really know what he's doing a lot of the time in a fight, he just wins because he understands that all it takes is one punch, however it wouldn't be like that with Ryu.

Winner evil Ryu