In shonen (and sometimes seinen) action anime series, the hero might win not through their own merit or cleverness, but by the sheer fact that they are the hero. Not all battles are won so cheaply, but sometimes, the hero will triumph over a much stronger foe simply because the hero must win, and move on to the next fight.
From the villains' perspective, they are getting cheated out of a victory that they should have won fair and square, and they lost because the plot won't let the hero die, or the plot gave the hero a brand new, exciting power at the last moment. Otherwise, the villain could have won if the plot didn't get in the way. Which 10 anime villains could have, and should have, carried the day?
10 Grimmjow Jeagerjaques, When He Fought Ichigo Kurosaki (Bleach)
Grimmjow Jeagerjaques quickly established himself as Ichigo Kurosaki's bitter rival, and he was determined to prove himself the "king" in fair but brutal combat. They clashed three times until their final duel inside Las Noches, where neither warrior held anything back.
By all rights, Grimmjow could have and should have won that duel, since his powers were greater while he was in his released mode. However, Orihime Inoue cried out to Ichigo, and Ichigo struck back with renewed strength and took Grimmjow down, despite the odds. If only Grimmjow had a cheerleader, too.
9 Muscular, When He Lost To Izuku Midoriya (My Hero Academia)
During Season 3, the ultimate melee showdown took place between Izuku Midoriya and the dreaded villain Muscular. Izuku was learning the ropes of One For All while Muscular could greatly enhance his strength and endurance with his muscle-based Quirk (hence his villain name).
As a whole, Muscular had the advantage in this fight, and he could have crushed Izuku and his incomplete One For All. However, Izuku got the ultimate power-up, the astonishing 1,000,000% Smash, and won the duel. If Izuku weren't the hero, things would have gone very differently.
8 Askeladd, Who Had To Cheat To Kill Thors (Vinland Saga)
The scheming Askeladd had been tasked with assassinating Thors on orders of Floki, the commander of the Jomsvikings. Askeladd is a skilled duelist, but he couldn't handle the sheer brutish strength of Thors, who temporarily defeated him in their fight.
Thors is honorable, while Askeladd is a cunning and devious person. So, Askeladd had to lose to Thors, to prove that he's the type to win by fighting dirty, rather than fighting clean. Otherwise, he might have triumphed over Thors fair and square when they crossed swords, protagonist style.
7 Amon, Who Lost To Avatar Korra (The Legend Of Korra)
Avatar Korra had a lot going for her, including her ability to bend fire, water, and earth all at once, and her confidence and courage in battle. But she struggled to bend air, and as of her final battle with the villainous Amon, she was still incapable of Airbending. Then Amon robbed Korra of her bending.
That should have been the end, but at the last second, in true shonen style, Korra gained airbending and used it to blast Amon away and defeat him. Under ordinary circumstances, Amon could easily have dispatched the Avatar and her friend Mako.
6 Lord Boros, Who Lost To Saitama (One-Punch Man)
Saitama, the caped baldy, drives the humor and themes of this anime with his extreme strength, and for the sake of comedy, he can defeat anyone with just one punch, no matter who or what they are. Then he met his match against Lord Boros, an alien warlord with similar strength.
If it weren't for Saitama's plot-based power, Lord Boros could certainly have taken on the entire Hero Association and win, due to his own immense strength and his ability to rapidly heal his wounds. But alas, Saitama's "I always win" power saw him through this fight, and Boros went down.
5 DIO, Who Lost To Jotaro Kujo (JoJo's Bizarre Adventure)
The Stardust Crusaders story arc featured the return of Dio Brando, now going by DIO. He had an incredible Stand named The World, and with its devastating punches and time-freezing ability, DIO should have been able to assassinate all of his enemies.
However, Jotaro Kujo was not only smart and tough, but adaptable, too. He inexplicably copied The World's abilities and began to move during frozen time, and with this power-up, he fought DIO on even terms and defeated him. But this victory should have been DIO's.
4 Haku, Who Lost To Naruto Uzumaki (Naruto)
Haku was a rogue ninja who used a kekkei genkai, creating ice mirrors that allowed him to rapidly move around and deliver needle-based attacks with impunity. Haku very nearly killed Sasuke Uchiha this way, and he could have killed Naruto Uzumaki, who had minimal skills and experience at the time. Except for one thing.
Being the shonen hero, Naruto housed a dark inner power, the Nine-Tailed Fox, that he could call upon during emergencies (Ichigo Kurosaki would know about that, too). With this fierce Fox power, Naruto turned the tables when he saw Sasuke's apparent death and defeated Haku. But otherwise, Haku could certainly have won.
3 Kenpachi Zaraki, Who Lost To Ichigo Kurosaki (Bleach)
This is another time in Bleach's story when Ichigo got a second wind, and then some. Ichigo lost fair and square to Captain Kenpachi Zaraki during their duel in the Soul Society arc, since Kenpachi was so much stronger. In fact, Ichigo didn't even have his Bankai at the time.
However, Zangetsu acted as Ichigo's hidden inner power, aside from the inner Hollow, and helped Ichigo inexplicably strike back and take down the vastly superior Kenpachi, simply because he trusted Zangetsu as a partner. Only a shonen hero can access benefits like that.
2 All For One, Who Lost To All Might (My Hero Academia)
The powerhouse villain All For One is the symbol of evil, and with his immense collection of Quirks, he can defeat any hero who gets in his way. Even the likes of Best Jeanist and Mt. Lady are easily crushed, but All Might can stand up to him.
All Might is indeed the world's top hero, but even he has his vulnerabilities, and he was at his weakest point during this climactic duel. All Might should have narrowly lost, but he needed to stick around to keep mentoring Izuku, so he used a last-minute power surge to eke out a narrow victory instead.
1 Fire Lord Ozai, Who Lost To Avatar Aang (Avatar: The Last Airbender)
Fire Lord Ozai really did have all the power in the world, since he had Sozin' Comet boosting his firebending to incredible levels. He could have killed Aang before the Avatar state came into play, but instead, being the sadist he is, he toyed with Aang and ended up facing Aang's energybending.
The entire concept of energybending appeared as a way for the pacifist Aang to defeat Ozai without killing him, and thus avoid his gruesome dilemma when it came to ending the war. Ozai could have won this otherwise.