Just one question for you first... Is Saitama actually listed as the one punch man? From what I recall the term "One punch man" is absolutely never used throughout the entire show... only in the anime name.
Anyway, I would have said that Naruto pushes the limits of power well beyond the limits of imagination and you can't help but wonder how someone like him and the general vanilla shinobi who exist only to die like flies during any attack come from the same village or even the same world... but now that I think about it, you are actually right, Madara is the most bad ass guy in that universe.
AllMight, meh, I think if he came toe to toe with Saitama then Saitama would repaint the walls with AllMight's insides and end up saying "What? Was that the grand match or when is that going to start? I need to get to the market before my coupons expire" .
Say what you want to but when a guy yells out his ultimate attack as "Consecutive normal punches" ... and puts in so little effort that he even forgets to yell (just barely manages to mutter it in fact) yet still leave behind only body parts and bloody mists... I think you can stop the competition right there. You have your winner...
Do you remember, the time when the people at the hero association start noticing him as one of the heroes who always finishes his opponent with one punch, hence the system of giving the nicknames that is there in the hero association is where the name One Punch Man is tossed for the first time.
And yeah regarding Madara he was only been able to kill was by Kaguya who betrayed him by stabbing from the back, I hope I remember XD. Otherwise no one was able to take him down not sasuke , not naruto, not guy sensei, no one.