Arguably the most important Quirk in My Hero Academia, One For All, is simultaneously the strongest oneーperhaps except for its diametric opposite. It exists solely to maintain balance in a world where empowered humans exist, which gives rise to the development of superheroes and supervillains.
One For All has been inherited nine times since its existence — All For One's younger brother being the first. The second, third, fourth, and sixth users have not been revealed yet, but the fifth is named Daigoro Banjo. The next three appear significantly in the story: Nana Shimura, Toshinori Yagi, and Izuku Midoriya. Taking the movies as canon, there is one other (temporary) wielder of One For All.
6 All For One's Younger Brother
The creator of One For All is the man who seeks to destroy it, All For One. He has a younger brother, only known by the family name of Shigaraki, who seems like he had been born Quirkless. To restore the balance, the elder sibling forcibly transfers a "Power Stockpiling" Quirk onto him, which then proceeds to fuse with a secret one that nobody knew existed.
The only capacity it possesses is transferring itself from one person to another, which is how One For All comes into being. Unfortunately, the younger Shigaraki is nowhere near strong enough to handle All For One's overwhelming force of nature, and his attempt to take him down fails. In the end, what happens to him is unknown, except he successfully passes down his Quirk before dying.
5 Katsuki Bakugo
In My Hero Academia: Heroes Rising, Midoriya somehow manages to "share" the One For All with his frenemy, Bakugo, who then proceeds to increase his own powers, especially his Explosion massively. In fact, when he uses his actual Quirk after this transfer, his blasts produce gigantic depressions in the ground, and the heat generated is capable of liquefying rock.
However, given that he has had zero time to train with One For All, and considering the amount of experience required to handle this tenacious Quirk, it can be assumed that Bakugo is not among the most powerful of its users.
4 Daigoro Banjo
The fifth user, Daigoro Banjo, is first seen in the manga when his original Quirk is manifested within Deku's body. He continues to teach the latest "avatar" in controlling and manipulating Blackwhip, which permits the formation of incredibly versatile pitch-black tentacles that are useful in terms of movement, climbing, trapping, and seizing.
Although Izuku has a lot to learn about this new technique, Daigoro is certainly capable of blending Blackwhip with One For All in incredibly unique ways. The story is yet to describe his powers in more detail, however.
3 Nana Shimura
Nana Shimura is All Might's mentor and presumably the first person to surrender her strength to a completely Quirkless individual. This shows that she believes in a person's strength of will rather than in the power of their Quirks. Further, the fact that she trains All Might to reach his full potential (as well as being close friends with the mighty Gran Torino) indicates that her own authority over One For All is nothing short of magnificent.
Not to mention that her initial Quirk, Float, helps her levitate herself without restriction. Further, All For One arrogantly claims that Shimura meets her end with disgrace, but she did not have One For All during her final fight, so her battle prowess cannot be judged from this statement.
2 Toshinori Yagi
Toshinori Yagi, or All Might, is probably the most powerful hero globally at the peak of his career. Interestingly, even after handing down the metaphorical torch, he has enough One For All in him actually to defeat his nemesis. His strength, resilience, agility, endurance are all completely on a different level, shown when Endeavor learns that there is no possible way for him to eclipse his rival.
Then there is the plethora of Super Moves in his arsenal, based on half the United States, and culminating in a "national" level Smash. After managing to put One For All in prison, Toshinori has none of his powers remaining (except the occasional ability to transform into his majestic appearance: All Might in all his glory.
1 Izuku Midoriya
There is no doubt that Midoriya will become the strongest person in existence when he figures out how to operate One For All at its fullest potential, something that no previous user has been able to do. In fact, All Might wonders if their predecessors' Quirks are starting to reveal themselves in Deku, which is later proven in the manga arcs. The young student is currently qualified to embody Danger Sense (from the unknown fourth user), which lets him perceive anything dangerous in his vicinity.
As already mentioned, he can use Blackwhip, inventing techniques such as Froppy Style, as borrowed from his classmate, Tsuyu Asui. Deku can also employ Shimura's Float, making his abilities all that more impressive. Finally, he is the only wielder of his Quirk who has been shown to enter an unconscious dimension where he can converse with his past incarnations.