Quirks are special abilities that exist within almost everyone in the world of My Hero Academia. Although quite rare once, Quirks have become increasingly common over the years and currently, around 80% of the population in the world of My Hero Academia possesses these meta abilities. All Might, the strongest Hero of all time, was considered to be the greatest Quirk user in his prime and much of it was down to the power of One For All that was passed down to him.
Eventually, All Might passed this power down to Izuku Midoriya, the protagonist of the story and, with time, Midoriya has gone on to make this power his own. When compared to the rest of the Quirks, One For All stands out as one of the strongest abilities to ever exist, and it is the very nature of its creation that's become the true source of its power.
The Creation Of One For All
The One For All is not a Quirk that always existed like All For One. Instead, it was created by a combination of two different Quirks. The creation of One For All dates back to several decades ago when Quirks first came into existence and people realized they could make use of these powers. The strongest of all Quirks, All For One, fell into the hands of a man named All For One. Using its powers, he was able to steal the Quirks of other people freely, stockpile them, make use of all those abilities for himself or distribute it among other people. Quite clearly, the power of All For One was incredibly dangerous.
His brother, Yoichi, was thought to not have a Quirk but, at the same time, he had a strong sense of justice. To turn his head, All For One gifted him a Quirk to stockpile energy. To everyone's surprise, Yoichi did, in fact, have a Quirk with the power to be passed down from one person to another. Together, the two Quirks combined and became what came to be known as the One For All. Yoichi was the very first user of this ability and soon, many others followed him, all with the aim to stop All For One.
Eventually, the seventh user of the Quirk, Nana Shimura, passed it down to All Might, who then went on to establish an era of peace using it and defeated All For One. Yet, All For One was alive and kicking and planned to claw his way back to the top. Realizing that he needed a successor, All Might chose Izuku Midoriya as the next person to wield this tremendous power and thus arose the 9th and the final user of One For All.
Why One For All Is The Strongest Quirk
The power of One For All surpasses that of almost every other Quirk to ever exist in the world of My Hero Academia. The basis for it to work is quite simple as all that it does is stockpile great amounts of energy and then release it, giving the user a drastic boost in all their physical attributes, be it strength, speed, or durability, among other things. The energy stored by this Quirk can either be distributed evenly throughout the body or be focused on one part of the body to be unleashed with tremendous ferocity.
Essentially, One For All is the only Quirk that can keep up with All For One. Not just because the Quirk is tremendously powerful but also because it is the only power that All For One can't steal using regular means as the Quirk itself was born to oppose All For One.
What makes One For All even more dangerous is that it keeps growing in strength after being passed down from user to user. Simply put, the first user of the One For All had the weakest version of the Quirk while the last user, Izuku Midoriya, has the strongest version of the Quirk as it was passed down to many others before him.
Due to the Quirk Factor, One For All has also stored the powers of all the other Quirks that the previous users have wielded. During the Joint Training Exam arc, Deku first awakened Blackwhip, the Quirk of the sixth user of One For All. With time, he's gained access to other powers as well, and they include, Smokescreen, Danger Sense, Fa Jin, and Float as well, which just goes to show that this Quirk is overpowered in every possible way.
If having access to multiple Quirks wasn't enough, all these Quirks are actually much stronger than they were under their original users. This is down to One For All's ability to get stronger after being passed down from user to user.
All Might, the greatest Hero in the world of My Hero Academia, was able to defeat All For One with just the power of One For All, meaning this Quirk already had enough power to defeat the greatest evil.
After being passed down to Izuku Midoriya, it has only gotten stronger and gained extra Quirk powers that weren't available to any previous user of One For All. It is quite easy to see that One For All is the strongest of all the powers to exist in the story. When Deku masters it to the fullest, there is no power that will be strong enough to match it. Possibly the only Quirk that would even come close to it is the All For One but even that would have to go through some significant upgrades to reach the level where One For All is slowly getting at.