Quirks are a phenomenon best described as superpowers within the My Hero Academia universe. Everyone is born with a different skill allegedly at random. However, the proficiency at which many are able to use it can vary dramatically, especially when one becomes over-reliant on their own special gift.
Some characters are so hopelessly dependent on their quirks that they cannot hope to fend off their opponents without them. By acknowledging those who have become too reliant on their power, we can determine their vulnerability against opponents like Eraserhead or All For One.
10 Todoroki
Shoto Todoroki was infamously dependent on his quirk, a fact that Eraserhead capitalized on when arranging for his test. After an icy glance from the teacher, the student was subdued almost immediately, leaving his partner solely responsible for his rescue.
Though he may be able to command massive glaciers and huge gouts of flame (rendering him one of Class 1-A's strongest students), his durability and fighting prowess in the absence of his quirk are disappointing. It pales in comparison to the likes of Bakugo and Deku, both viciously capable even without their respective powers.
9 Kirishima
Though Kirishima is a capable melee fighter even without his quirk, he cannot adapt to a battle expediently because his "Hardening" is integral to the way he defeats his opponents. Since he can make his body virtually indestructible, he allows his enemies to exhaust themselves against him before delivering a brutal punch.
In the absence of his power, his usual tactics would work against him, defeating the intrepid hero almost instantly. His excessive reliance on his defense would spell his downfall since he does not have remarkable agility.
8 Kurogiri
Kurogiri is able to secrete an immense fog around him that can teleport himself and his allies. It is known as the "Warp Gate," a skill responsible for salvaging the League of Villains' efforts on multiple occasions.
However, he was never displayed as a particularly competent fighter outside this special gift and served a support role in much of Shigaraki's endeavors. When Gran Torino had apprehended him, he was defeated by the senior hero almost immediately in spite of Gigantomachia's intervention on his behalf. This displays his ineptitude as a fighter, one cultivated through his frequent aversion to direct confrontation.
7 Yaoyorozu
Momo's "Creation" quirk is frequently underrated. Though it does not contribute to her martial skill, she can produce anything from her body that she knows the composition of. This renders her a peerless support hero, especially to the likes of Sato and Kaminari (who require sugar and lead blankets in order to work effectively).
Though she was able to outsmart Eraserhead and liberate Todoroki, this was only because he could not keep an eye on both his targets simultaneously. In an even fight, she would be helpless without her quirk, especially if she did not have time to create anything before it was taken away from her.
6 Overhaul
Overhaul was the clandestine leader of the Yakuza and a man who created quirk nullifying bullets (as demonstrated through his battle against Mirio). His quirk allowed him a versatile number of options, capable of rearranging anything he touched.
However, he is hopeless in a fight without it, proven through how Lemillion had dominated him in a melee engagement. All Chisaki needed to do was land a single punch against the hero, failing comically in his efforts to bring him down.
5 Muscular
Muscular is a large and dominating fighter that uses the strands of meat on his body in order to assist him in battle. Like Kirishima, his techniques revolve around absorbing and ignoring abuse rather than dodging it, capable of withstanding the might of Deku's One For All and forcing him to overdo it in order for success.
Though he may be physically powerful even without his quirk, his defenses would suffer immeasurably. Considering that he would no longer be able to manipulate the strands on his body to bring his enemies down, his raw force could not compensate for his new deficiencies.
4 Mt. Lady
As her name suggests, Mt. Lady is capable of becoming a giant and would frequently employ this skill to stop opponents in their tracks. It also allowed her to save dozens of civilians at once by shielding them with her own body.
A consequence of her quirk is that she has become almost exclusively reliant on fighting opponents much smaller than her. Accordingly, should she have her power removed, the hero would not be able to excel in a direct fight on account of her inexperience against opponents of equal size.
3 Fatgum
Though Fatgum's corpulence may seem unappealing for hero work, his quirk allows him to turn it into his greatest weapon. By burning calories and absorbing the force of his opponents' attacks, he is capable of reflecting their damage back at them and magnifying it.
Unfortunately, the hero's body shape would debilitate him without his quirk, and it would render his defense fighting style completely obsolete. In the absence of his gift, Fatgum is both unable to defend himself and too slow to manage any problems he might encounter.
2 All Might
All Might is unique since he has actually lost his quirk. In its absence, he suffers from more than simply a combat disadvantage; he becomes the withered husk that he had hoped to keep from the prying eyes of Japan's public.
Toshinori was aware of the fact that he could not do hero work after his power had been expended. It would prompt his retirement as the Symbol of Peace - a massive decision, considering the resilience and determination that he had displayed in the past for saving his people from the League of Villains.
1 All For One
All For One is among the strongest villains in My Hero Academia and the first to publicly humiliate All Might during their battle at Kamino. Stockpiling dozens of valuable quirks, his versatility and strength renders him one of the most dangerous men in existence.
However, he is only able to function because of his quirk and is useless without it. If he can't use his special power, he loses his ability to perceive the world around him since he lost his original eyes in a battle against Toshinori. Fierce as he might be, he is deceptively dependent on the gift he was born with.