My Hero Academia is a massively popular superhero anime that's known for its incredible diversity of characters and powers, called quirks. These abilities range from useful but unimpressive skills, like being able to manipulate and draw in 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 may not be 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 - All For One
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 steal 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 these stolen quirks to anyone he chooses.
One For All - Izuku Midoriya And All Might
One For All is actually 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, and strengthens his bones and muscle fibers, it becomes a far more useful ability.
Rewind - Eri
Eri possesses one of the most dangerous, volatile quirks in the entire My Hero Academia universe. Her ability, Rewind, allows her to manipulate her target's biological clock to turn back time, but only as it relates to the target's physical makeup. This quirk allows her to bring someone's body to a state before it was injured, removing the injury.
On the flip side, she can also bring back old injuries that have since healed. She can restore someone's youth, but she can also bring them back in time to infancy, or before they existed in any form – a fatal application of the quirk.
Eri is only six years old, and she's been manipulated into using her quirk for nefarious purposes. Hopefully, with the help of the staff at UA, she'll learn to use it in positive ways.
Half-Cold Half-Hot - Shoto Todoroki
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 to use them for things like breaking down doors and using the ice for transportation.