In a society where superpowers are not only normal but common, it's only natural that these abilities would be scientifically analyzed. Many of My Hero Academia's most powerful characters are so strong not only because of their innate strength, but their deep understanding of how their quirk functions and constant pushing of their power's limits.
Throughout the series, the creators have shed some light on the specifics of the biological processes of quirks, investing readers in the lore of the powers as well as the characters who wield them. While plenty of quirks are well understood, others still seem to push past the boundaries of what science would allow. Just how their wielders' bodies shape their power may be revealed later, or simply remain a mystery to audiences throughout the rest of the series. Only time will tell.
10 Makes Sense: Creation (Momo Yaoyorozou)
Using the lipids/fats that she consumes and are a part of her body, Momo can create nearly any object she can think of. To do so, she has to know not only how the object functions and what it looks like, but its molecular structure. Her limits can be reached if she outputs too many things, burning too much fat and causing her to become anemic. It is this restriction that pushes her quirk into the realm of believability. While this power would be much too complicated for someone with an average memory to handle, Momo's genius-level intellect and fantastic memory allow her to make the best of it.
9 Confusing: Dark Shadow (Tokoyami)
While most emitter quirks are centered around the control of an element, or the creation/alteration of physical things, Dark Shadow is unique in that it is seemingly sentient. Just how Tokoyami ended up with another being inside of him is never really explained, nor is any plausible biological explanation given for it. This quirk seems more like a mystical curse because of this, and it makes it harder to really measure Tokoyami's growth as a result.
8 Makes Sense: Earphone Jack (Jiro)
Compared to many of the combat focused students at UA, Jiro's Earphone Jack is a mundane yet welcome departure. The audio jacks attached to her ears are fantastic at picking up noise or vibrations, making her great at surveillance, but they can also amplify her own heartbeat. Combined with her costume's Stereo Boots, she can also use them to blast out powerful soundwaves. Her abilities, unlike many others', make sense immediately. She detects vibrations with special appendages and amplifies the sound of her heartbeat using a special device.
7 Confusing: Whirlwind (Inasa Yoarashi)
Inasa's quirk had the potential to make some sort of sense. If he emitted winds from some body part or had wings to sweep air around him, it would have felt more believable. Instead, his power borders on straight-up magic, manipulating wind around him with no obvious effort on his part other than concentration.
This separates him from other emitters, like Todoroki, because using his quirk doesn't affect his body if used for too long. While Inasa has one of the strongest quirks the show has seen, it would have benefitted from a more concrete explanation.
6 Makes Sense: Explosion (Bakugo)
A fan favorite due to his character growth Bakugo's quirk is explained early on in the series. He can secrete a nitroglycerin-like sweat from his palms that he can detonate at will, creating explosions of various sizes. He can use his hands as jets to move around or cannons to devastate the opponent, but there's a drawback. The kickback from using his power can wear Bakugo out, even damage him if he's not careful. Both the biological background and the limits placed on his body make Explosion one of the coolest and most believable quirks in the series.
5 Confusing: Fiber Master (Best Jeanist)
Best Jeanist's power is a confusing one, to say the least. Having complete control over all types of fabric/fiber is a poorly defined ability, as its limits are still unknown. Could he control muscle fibers? Are there fabric types that he controls better? It also doesn't help his case that there's no real strain on his body other than concentration. This lack of definition is probably due to the limited screen time he received, and hopefully, if he recovers from his fight with All-For-One, we'll learn more about his strange quirk.
4 Makes Sense: Overhaul (Kai Chisaki)
A quirk that completely disassembles and reassembles solid matter, Kai Chisaki's Overhaul is by far one of the strongest abilities seen on an anime character. All it takes is one brush from his hands and his enemies are doomed. But therein lies his all-important limitation.
Like Bakugo, his hands are where his quirk manifests, so by immobilizing or removing his hands, Overhaul cant be used. Exactly how his molecules interact with others isn't known, but the greater extent of his power is well explained enough to inject a sense of realism into his crazy strength.
3 Confusing: Fierce Wings (Hawks)
While the wings on his back are self-explanatory, Hawk's feathers are what stand out as an unexplainable phenomenon. They zip around wherever Hawks needs them to be, defying physics in a crazy way. They also have the structural strength to support an entire human on a single feather. Not only that, but he can sense their surroundings and make deadly blades from them as well. All of these abilities culminate in a confusing bond between Hawks' mind and his feathers. Fortunately, since he holds such a prominent position in the Hero Registry as the Number Two Hero, it's possible the audience will learn more soon.
2 Makes Sense: One-For-All (All-Might, Midoriya, and Others)
By willingly giving a piece of their DNA to a recipient, the bearers of One-For-All impart both the stored power of past users, as well as the ability to pass along One-For-All. Since its inception, the quirk has gone through nine users, with Izuku Midoriya being the latest bearer. While the actual amount of power within him is unknown, the ability to impart genetic memories and acquired strength is believable enough.
1 Confusing: All-For-One (Shigaraki)
While seemingly simple, the ability to both steal quirks and impart them to others comes with a lot of questions. All-For-One has been shown to possess at the very least sixteen quirks. With the amount of strain that a fraction of that puts on various Nomu, Shigaraki's DNA must be in shambles, and his body constantly strained by the power residing in him.
However, this doesn't seem to be the case, as he's shown wielding the powers he has nearly effortlessly. It's a good thing he'll most likely make a comeback as the final boss of the series, meaning audiences will get to see the extent of his power pushed even further than in his battle with All-Might.