His skills and techniques alone made him the most powerful ninja during the height of the ninja golden age if you will, since that was an era of constant battles between clans it produced the greatest and most skilled of shinobi. And for Both Madara & Hashirama to have stood atop of all them equally by leaps and bounds says alot of Hashirama's prowess and skill
add on top of an already unparralell base power, Hashirama also had all 9 bijuu in his possession at one point and had the power to control them all like pets. That power right there at his disposal makes him untouchable cause if he summon just 2 of them, game over. Now like a dope he gave up all that power and just handed them over to the other villages I guess to play fair...but at that time when he had them all...no one could mess with him
That's why I'd have to agree and say I think he's the best behind the SOT6P
Rinnegan Madara pwns Hashirama any day U_U