My Hero Academia is a massively popular superhero anime known for its incredible diversity of characters and powers, or quirks. These abilities range from useful but unimpressive skills, like being able to manipulate and attract small objects, to mind-blowing talents, such as the capability to rewind a target's biological clock.
As is the case with most anime, some of the abilities on display in the show are vastly stronger than others. Power is not the be all and end all when it comes to how useful a hero or villain is, but it certainly separates the exceptional ones from the pack. There's Shoto Todoroki's Half-Cold Half-Hot, which allows him to simultaneously control fire and ice, and Best Jeanist's Fiber Master, which gives him total dominance over anyone wearing clothes. Then, of course, there's One For All, a quirk used by Izuku and All Might that bestows a burst of unimaginable strength.
There are countless incredible quirks in My Hero Academia and, in a show that prides itself on continuously introducing dominant characters, it can be tough to draw the line and single out the best of the best. However, it's clear to the audience who stands tallest among the heroes and villains.
All For One is a major villain in My Hero Academia, so it stands to reason his quirk would be formidable. All For One's quirk, with which he shares his name, allows him to permanently take abilities from other people, leaving them quirkless. Not only can All For One use these skills himself, he can also combine them into new techniques that are far more powerful than the original quirk alone.
What's more, he can also give the quirks he has taken to anyone he chooses.
One For All is a combination of two quirks. One allows the user to accumulate huge reserves of raw power in their body, and the second grants the ability to transfer the quirk from person to person. The first part of One For All is what propelled All Might to the top of the professional hero world, and the second one is what allows him to pass his quirk on to Izuku Midoriya at the beginning of the series.
While the quirk affords its user incredible physical strength - enough to break down a wall or defeat an opponent with a single punch - it can be dangerous in a body that isn't powerful enough to handle it. When Izuku is first getting used to his new quirk, he breaks his bones every time he tries to use it. As he develops techniques for evenly distributing his powers, as well as strengthens his bones and muscle fibers, it becomes a far more useful ability.
Overhaul is one of the most over-powered quirks in My Hero Academia. Kai Chisaki can break down anything he touches into its most elemental components, and then he's able to rebuild it into any form he sees fit. He can use it to wipe out an opponent's body completely, bring back an ally from the brink of death, or cure a person's chronic illness. He's difficult to track, as he can destroy and then reform his surroundings as he passes them, making them look indistinguishable from moments prior.
He can even break down someone else's body and graft part of it to himself, effectively taking their quirk or other abilities. Overhauls's only weakness is that it's localized in Chisaki's hands, so if his hands were removed, the quirk would no longer function.
Adopting both his father's fire powers and his mother's ice abilities, Shoto Todoroki technically has two quirks in one. He produces fire with the left side of his body, and ice with his right, resulting in a quirk called Half-Cold Half-Hot. Because Shoto has a poor relationship with his fire-using father, he's neglected the possibilities of his left side for years and hasn't quite mastered using both elements simultaneously.
As he develops his quirk, he's able to not only combine his two abilities, but use them for things like breaking down doors and using the ice for transportation.