The Ultra Analysis stats give All for One perfect stats in everything, 6/6 S+ for power, speed, intelligence and technique, when All Might only has 6/6 S+ in power and speed, and 6/6 S in intelligence and technique. Is All for One more powerful than All Might?
I think One For All is stronger.
Think of it like this: All For One is definitely weaker like All Might. Supporting this fact is that he still hasn’t healed, and is wearing a life-support mask. When they fought the first time, he messed up All Might's stomach really badly. In the second fight, All For One only held him back with his hands.
And when class A and class B were attacked on the campsite, Muscular lost to Midoriya who has One For All. When Midoriya passed by Spinner during the attack, he claimed that Muscular was the second or first strongest in the group.
Dabi claims that Tomura was weak for sending a bunch of rookie villains and that Dabi brought a bunch of hard-to-defeat villains.
Therefore, I think One For All is stronger.