5-STRONGEST: MATTER CREATION-BASED QUIRKS
When it comes to supernatural abilities, those who can create objects/matter from nothing are truly powerful. BNHA has some great Quirks that showcase the benefits of such an ability. Momo Yaoyorozu from class 1A of the U.A Academy, is probably the prime example with her Creation Quirk because the smarter she gets, the more terrifying weapons she can create.
Space Hero, (Thirteen) is also a more unique example because of the innate scare factor associated with Black Holes. When it comes to pure strength with creation, no one is stronger than Endeavor's son, Shoto Todoroki also called, "Icy-Hot", is popular for a reason.
4-STRONGEST: PHYSICAL ENHANCEMENT QUIRKS
Next, we've got the Physical Enhancement Quirks. These are things like super strength and mach speed. One for All would be the best example but that feels a bit like cheating, so let's talk about Muscular instead.
This Villian can stack muscle fibers on top of each other seemingly infinitely, making him the end all be all powerhouse. Of course, there are others like Sugar Rush from 1A, Kendo Rappa from the Overhaul Arc, or even Sorahiko, All Might's teacher.
3-STRONGEST: CHANGEABLE BODY-STATE QUIRKS
Any quirk that can mess with either someone else's or their own body structure is scary powerful. Mirio Togata can phase in-and-out of anything or anyone, Shigaraki can turn people into dust, but no one of them are probably more powerful than Kai Chisaki.
The guy can explode people just by touching them with his finger, then fusionnate their remains into a wall. Basically, it's like Horikoshi took the healing capabilities of Chiyo Shuzenji ( Recovery Girl ) and combined them with the destructive power of Shigaraki's Decay.
2-STRONGEST: ANIMAL-BASED QUIRKS
The more general a superpower, the better. If it's vague, then it's potential for strength is only tied to the creativity of the user. Animal-based quirks seem to be like this in MHA. Everyone's favorite girl Tsuyu Asui, also called Froppy has the "Frog" Quirk. This means anything a frog can do, she can too.
Which apparently, based on powers she shows later on in the anime and particularly the manga, means it applies to all species of frogs! There are also examples of the opposite like Kugo Sakamata ( Gang Orca ) whose Quirk is tied specifically to the Orca variant of whales, but even then, he's still top-notch as a pro Hero! Also, don't even get me started on heroes like Centipeder or Ryuko Tatsuma, who we've seen absolutely destroy villains in the manga with such facility.
1-STRONGEST: COPY QUIRKS
When you're in a world filled with destructive supernatural abilities, being able to copy or steal Quirks is too overpowered. While All for One is probably the best example for this, let's talk about the others Heroes of Boku No Hero Academia. People like Class B's Phantom Thief is strong enough to copy any quirk and even stack multiple! Suneater can't copy Quirks but can take on attributes based on the ingredients in the food he eats.
Lastly, there's Jin Bubaigawara, also called Double with its Emitter quirk, and Ectoplasm, who can create clones of themselves, which on its own isn't scary. In the Manga, Double literally overtakes an entire city by flooding it with clones like a tsunami, as you will probably see during the 4th Season!