Summary
Gear 5 is the ultimate transformation of LuffyinOne Piecethat has proven to be exceptionally overpowered. Luffy attained this transformation after being knocked into a near-death state by Kaido. His Devil Fruit ended up Awakening and he gained this all-white transformation which strengthens his body greatly and also gives him more freedom as well as the ability to pull off impossible things as long as he can imagine them.
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It is quite clear that Gear 5 might be the strongest transformation in the entirety ofOne Piece. However, there are a few others that can certainly rival it to a degree. While the vast majority of these transformations cannot best Gear 5, they are still impressive enough to hold their own against Luffy, at least for a while.
5Big Mom’s Life Span Consumption Transformation
Big Mom is one of the former Four Emperors of the Seaand an exceptionally powerful pirate. In her life as a pirate, she had attained a bounty of 4.388 billion berries, which just goes to show that she was among the elites. Big Mom had several abilities up her sleeve and one of them was the ability to grow tremendously and strength and size by consuming her own lifespan.
She took this transformation on during her fight against Kid and Law and went on to largely dominate this fight. This increased her strength as well as durability and her endurance by a great degree. Even a barrage of powerful attacks that dealt internal damage could not put her down. In fact, Kid and Law were not able to knock her out at all. The best they could do was throw her off Onigashima and Big Mom then signed her own downfall by accidentally making some incredibly powerful bombs land on herself.

4Bloodthirsty Drunk Kaido
Kaido has several powerful transformationsin his locker and while his standard hybrid form is powerful enough to rival Gear 5 on its own, he has even stronger forms under his belt. One of them is the bloodthirsty drunk state that he attains after entering his Shuron Hakke phase. By consuming a large amount of alcohol, it appears that Kaido can up his strength even more.
He gains access to unique transformations that are quite similar to Devil Fruit Awakening themselves and might even be classified as those. One specific transformation sees him gain a much bigger body and a more of an animalistic appearance, with a bigger nose, sharper teeth, and bigger arms, among other things. His physical attributes are boosted to a sky-high level when he takes this form and his attacks become even more powerful than ever before. Kaido also takes a purple hue when he takes this transformation.

3Kaen Daiko
Kaen Daiko, also known as Flame Dragon Torch, isthe strongest transformation that Kaidohas under his belt. This form sees him transform into a massive dragon that can engulf an entire island with ease. This dragon is made out of incredibly hot flames resembling magma, and the tremendous heat that this dragon possesses is enough to vaporize an island by merely passing over it.
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Quite clearly, the strength that this dragon possesses is tremendous and even a single hit from it would be lethal. Kaido used this transformation in his fight against Luffy at the very end. Unfortunately for him, Luffy knew how to damage his enemies without making contact with them. By using Ryuo, he channeled the power of his Bajrang Gun from a distance and utterly demolished Kaido.
2Gorosei Devil Fruit Forms
The Gorosei are some of the mightiest figures in the world ofOne Piece.Until recently, fans believed them to be just figures who oversee the politics of the world. However, the Reverie arc changed their perception of these characters when it was revealed that these powerful beings are also fighters.
They are termed Warrior Gods, which means that the level of strength that they possess is likely very high. Their Devil Fruits were also teased to the fans. While their actual fruits were not revealed, all of them seem to possess Zoan-type Devil Fruits, as all of them took on unique transformations. Their fruits are most definitely extremely powerful and these transformations would be strong enough to rival Gear 5.

1Imu’s Transformation
Imu is the sovereign of the entire worldand most definitely the ultimate villain inOne Piece.Not much has been revealed about them, other than the fact that they are a relic from the Void Century and have been alive ever since then. At the same time, it was also revealed that they belong to the Nerona Family and, at one point, they were among the 20 Kings who joined forces to oppose the Ancient Kingdom.
Somehow, Imu ended up becoming the sovereign of the world. Currently, they are the de facto ruler of the World Government and a lot can be expected from this character towards the end of the story. Naturally, no character who finds themselves in this position is going to be weak. In the Reverie arc, fans got a look at the silhouette of their Devil Fruit power and it certainly looks terrifying. The fact that even the Gorosei bow down to Imu only means that their power, especially when taking on the Devil Fruit transformation is going to be immense. As such, they can certainly rival Gear 5 Luffy.

