Every now and then, an anime hero would lose all their powers for the greater good. More often than not, this is done to drive home a very impactful sacrifice or to teach a valuable lesson about how gaining power for powers' sake is bad.
Take the deliberately overpowered All Might from My Hero Academia for example. Halfway through the series, he loses his superhuman powers, cementing the end of his heroic generation while making way for the next.
WARNING: Spoilers ahead.
10 My Hero Academia — All Might Barely Survived His Fight Against All For One
All Might may be the strongest hero alive, but even he was almost no match for All For One, the world's worst supervillain. Although All Might won their first fight and injured All For One so grievously that regenerative Quirks couldn't heal him, the Symbol of Peace sustained his own injuries that severely affected his superhero career.
Not only did All Might lose a lung and his stomach, but his body was slowly rejecting his Quirk, One For All. Before it completely vanished, All Might successfully passed One For All to none other than Izuku Midoriya. His remaining power finally ran out when, during his final fight with All For One, All Might used up its last vestiges to try to beat the villain for good.
9 One Piece — Whitebeard Went Up Against Blackbeard
While Blackbeard isn't the strongest pirate in all of One Piece, he more than makes up for it with his devastating Devil Fruit. Blackbeard ate the Yami Yami No Mi, and one of its powers lets him neutralize anyone's superpower as long as he's holding them. He used this to devastating effect against Whitebeard, but it didn't do much.
Unfortunately for Blackbeard, Whitebeard was a lot more than his earthquake powers. After Blackbeard temporarily nullified his powers, Whitebeard wasted no time in beating up his cocky foe with his fists and spear. Blackbeard only escaped certain death when his crew fired all their guns at Whitebeard in the nick of time.
8 Inuyasha — Inuyasha Becomes Human Whenever There's A New Moon
As powerful as he is, Inuyasha has one weakness that he's never told to anyone. Every time a new moon emerges, Inuyasha loses his powers and becomes a normal human. This was a side-effect of being the son of a demon father and a mortal mother, and it's something Inuyasha despised. In fact, it's what drove him to steal the Jewel of Four Souls.
Because it's been said that the Jewel could turn a half-demon into a full-fledged one among other things, Inuyasha tried to steal it from under Kikyo's watch. Instead, they fell for each other. Just when Kikyo was about to use the Jewel to turn Inuyasha into a human, he stole it which then led to Kikyo casting a sleeping spell on him that lasted for 50 years.
7 Boruto: Naruto Next Generations — Naruto Uzumaki Lost His Godlike Powers After Kurama's Sacrifice
One of the most recent developments in the legacy series Boruto: Naruto Next Generations was also the saddest. During the fight against Isshiki Otsutsuki, Naruto used the powerful Baryon Mode assuming that it would drain his life force. Instead, it took Kurama's, and he never told Naruto because he knew his host wouldn't allow it.
Before dying, Kurama explained that while Naruto would live on, he would lose his godlike abilities now that the Nine-Tailed Demon was out of his body. That said, losing his powers wasn't a problem for Naruto. His bigger concern was losing Kurama, who went from a living curse he detested to one of his dearest friends.
6 Kill La Kill — Ryuko Matoi Was Saved By Senketsu
Even if she has some Life Fibers in her body, Ryuko isn't all that powerful. But when she wears the sentient Kamui named Senketsu, Ryuko becomes an unstoppable warrior capable of besting Honnoji Academy's best fighters. In fact, she and Senketsu even defeated a Life Fiber-empowered Ragyo Kiryuin in the final fight.
Thing is, Ryuko and Senketsu defeated the fusion of Ragyo and the Primordial Life Fiber in space. After that, Senketsu used the last of his powers to let Ryuko land safely on Earth. With all Life Fibers (and Senketsu) now gone, Ryuko became a normal high school girl, but she didn't mind since she and Senketsu at least said goodbye to each other.
5 Bleach — Ichigo Kurosaki Used All Of His Powers To Defeat Sosuke Aizen
By the end of the Fake Karakura Town Arc, Sosuke Aizen was so powerful that even the Shinigami using up everything they had wasn't enough to stop him. Ichigo took this to a literal extreme, as his most powerful attack - the Final Getsuga Tensho, which turns him into a being of pure reaitsu - came at the cost of his Shinigami powers.
After delivering a decisive final blow on Aizen, Ichigo's powers slowly faded away, but not before he was able to say farewell to all his Shinigami friends - especially Rukia Kuchiki. Whatever finality this moment may have had was undone by the succeeding Tale Of The Lost Agent Arc, where Ichigo recovered his powers by becoming a Fullbringer.
4 Hunter X Hunter — Gon Freecss Lost His Nen To Annihilate Neferpitou
The cat-like Chimera Ant Neferpitou was one of the most insurmountable foes Gon ever faced, and he went above and beyond to defeat them. To avenge Kite and slay Neferpitou, Gon summoned all the Nen he would use now and as an adult, rapidly aging himself and cutting years off his lifespan. As badass as this sounds, it actually wasn't.
A core theme of the Chimera Ant Arc was Gon losing his humanity for the sake of his pride and victory. A formerly weak Gon got what he wanted when he brutally murdered Neferpitou with his newfound immeasurable power, but it came at a steep price. After winning, Gon lost his ability to use and even see Nen, and his longtime friend Killua Zoldyck parted ways with him.
3 Fullmetal Alchemist: Brotherhood — Edward Elric Traded His Gate For His Brother's Life
The final fight against Father resulted in many casualties, including Alphonse. The younger Elric brother gave his life to help Ed regain his right arm, which he then used to end Father once and for all. Rather than use a Philosopher's Stone to resurrect Al, though, Ed used human transmutation on himself to bring back Al in exchange for his Gate.
A person's Gate lets them be an alchemist, meaning Ed sacrificed his powers to bring Al back. Ed's choice wasn't just correct but so excellent that it impressed The Truth, who commended him for realizing that the human experience and maintaining connections was more important than absolute power or controlling everything.
2 The Disappearance Of Haruhi Suzumiya — Haruhi's Godhood Was Sealed Off By Amnesia
Without exaggeration, Haruhi is probably one of the most popular and well-known goddesses in anime. Over the course of the mainline anime The Melancholy of Haruhi Suzumiya, Haruhi never realized that she was a godlike being who rewrote reality as she saw fit, even if unconsciously. That came to a temporary end in the finale movie.
After Haruhi subjected the SOS Brigade to the Endless Eight time loop, a fed up Yuki Nagato retaliated by using her alien powers to erase Haruhi from of everyone's memories, save for herself and Kyon. While an amnesiac Haruhi was alive and well, she was in another school and had no desire to live an extraordinary life, keeping her powers at bay.
1 Hell Girl — Ai Enma Remembered Her Humanity & Defied Hell's Orders
Before she became the ferryman between the living world and Hell, Ai was an ordinary girl who was unjustly lynched by a town that believed in human sacrifice. After serving the Master of Hell for 400 years, Ai willingly suppressed her humanity and memories. However, her stoic façade cracked in the anime's second season.
When bringing an innocent Takuma Kurebayashi to Hell, Ai turned the boat around to save him. The price for this disobedience was reverting to her human self and losing her supernatural powers, leaving her vulnerable to a vengeful mob. Though she was eventually resurrected for the sequel season, Ai died free in that moment.