In the world One piecethere are many different Devil Fruits that fall into one of three categories; Paramecia, Logia or Zoan types. With the latter, the user can transform into various types of animals, although there are also much rarer versions of the Zoan Fruit that allow transformation into extinct animals or, most interestingly, even mythical creatures and gods.
They are called Mythical Zoan Devil Fruits and tend to be the strongest Devil Fruits of all One piece. As such, some of the most powerful characters in the setting are Mythical Zoan fruit users, using which they have some insanely powerful attacks.

One Piece: Best Armor Haki Feats Roronoa Zoro, Rated
Zoro is a powerful Armament Haki user and his numerous post timeskip feats prove it.
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Yamato's Namuji Glacier Fang matched Kaido
Debut Arc: Wano
Yamato debuted in the Wano arc and proved to be an incredibly powerful character, especially in their battle with Kaido. For a long time, they even managed to match the strongest creature in the world. In this fight, one of the strongest moves Yamato used was a direct attack on their incredible Mythical Zoan Fruit.
Namuji's Glacial Fang allowed Yamato to create many icicles around their body and then fire an ice breath attack that was so powerful that it matched Kaido's legendary Blast Breath, confirming Yamato as an exceptionally strong character with a very powerful Devil Fruit.
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The Sengoku shockwaves suppressed the Tremor Fruit
Debut Arc: Marineford
It's no secret that Whitebeard's Gura Gura no Mi is the most powerful Devil Fruit in the entire series, as it is capable of destroying the entire world. Sengoku is also the user of the legendary Hito Hito no Mi, Model: Daibutsu Mythical Zoan Devil Fruit, which has the power to create extremely powerful shock waves.
Although we never got a named attack from the former fleet admiral, the shockwaves he created were strong enough to almost completely stop the shocks caused by the Gura Gura no Mi. This was after Blackbeard stole the fruit from Newgate's body, making Sengoku one of the strongest characters of the old era.
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Kaid's flame dragon torch made him untouchable
Debut Arc: Wano
To this day, Kaido is still remembered as possibly the greatest enemy Luffy has ever faced. Many One piece fans tend to keep him at the top of the list because of his insane performances. Although he has shown some incredible techniques in his many fights, such as Thunder Bagua or Blast Breath, his greatest attack is his insane Flame Dragon.
Kaido used his Devil Fruit's fire breathing ability to cover his entire draconic body with fire hotter than the one he uses with his Blast Breath, a notable difference since Blast Breath can destroy entire mountains. With the Flame Dragon Torch, Kaido grows exponentially and gets a massive boost to his already incredible speed and power.
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Thorheim is one of Loki's strongest attacks
Debut Arc: Elbaf
The Elbaf Arc revealed the existence of the legendary fruit Nidhogg, currently in the possession of Prince Loki of Elbaf. Through this mythical Zoan fruit, Loki transforms into a towering dragon the size of an island, making his power one of the most ferocious in the entire world. One piece settings.
In his brief altercation against Im, Loki used a breath attack similar to Kaido's Blast Breath called Thorheim, only Thorheim was much larger and much stronger. The size of this attack and its blast radius was so huge that even Imu was unable to escape, proving just how formidable Loki's power is in his Nidhogg form as he even damaged the villain significantly.

After 17 years, One Piece reveals the 3 most powerful mythical Zoan Fruits
The mythical Zoan fruit was introduced 17 years ago in One Piece, and in the ultimate Elbaf, Eiichiro Oda lined up three of his strongest.
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Imu's Omen/Maki is a terrifying mystery
Debut Arc: Elbaf
Few expected Imu to make his full debut in the Elbaf arc, but when he did, the entire power scaling of the series caused an uproar as his power was completely off the charts. One of the biggest reasons for this legendary power was Imu's mysterious Omen/Maki ability.
Although very little is currently known about it, it can be seen that through the Mark of his Devil Fruit, Imu can greatly empower anyone of his choice to an unprecedented degree, not only increasing their overall strength, but enhancing their abilities even more. This is a power that is even greater than his Domi Reversi and many believe that Maki is an evil version of Haki.
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Bajrang Gun is the most powerful mythic Zoan attack
Debut Arc: Wano
In his Gear 5 state, Luffy gains access to minor reality-warping abilities that allow him to do many fantastic things. One of the most obvious is his ability to change his size at will, which he did every time he used his legendary Gear 5 state. The most powerful attack that Luffy can do in this form is his Bajrang Gun.
Much like Loki's Thorheim, part of the reason why the Bajrang Gun is arguably the most powerful attack of all One piece is due to his sheer size making it impossible for anyone to dodge effectively. Expanding the size of his fist to the size of an entire island and infusing it with Advanced Conqueror's and Advanced Armament Haki, Luffy then unleashes an attack with unparalleled destructive power.
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