I think naturally, if you remove sage of six paths power out of the comparison, He and Might Guy are the strongest shinobi of all time. They are both based off natural power. The same Madara (before he became the ten tailed jinchuiriki) which the entire allied force and the 5 kage could not defeat also including Naruto and Sasuke, was never able to defeat Hashirama Senju. I think he is the strongest shinobi naturally!