Linear logic doesn't work in Naruto. i.e. Ninja 1 > Ninja 2, and Ninja 2 > Ninja 3 , so Ninja 1 > Ninja 3.
It's all about match ups, and to be the most "powerful" Shinobi in history, you either mean would not lose a fight to any shinboi, or has the most raw power of any shinobi, both are wrong.
As well as this, Nagato is leagues above 6 paths of pain, undoubtedly, and I also strongly believe Nagato would win Hashirama in a fight.
And another thing, Mokuton can suppress chakra so saying that because he can beat a Bijuu on his own, and others can't means he is stronger, no, it means he is better suited to that fight.
I do believe he is one of the strongest ninja ever, but I must say he is the most revered.