The Straw Hat Pirates are a powerful pirate crew in the world of One Piece who are currently sailing the seas with immense pedigree, being one of the Yonko crews. This group of pirates is exceptionally strong, being led by the Yonko, Monkey D. Luffy. Under Luffy, there are his Nine Great Officers, all of which are incredibly skilled.
While there are several fighters among the Straw Hats who don't rely on complex abilities and transformations, there there are also those who power up through these unique transformations that take their abilities to the very next level. These transformations have made One Piece fights iconic and they continue to evolve even today.
6 Franky - General Franky
Although not exactly a full-blown transformation, Franky is at his strongest in his General Franky form. The General Franky is a giant iron mecha body created by him during the 2-year time skip. In times of need, Franky jumps into its cockpit and starts controlling this giant body as if it were a massive robot. The General Franky is made with Wapometal, which is a shape-retaining metal, meaning that quite a lot of damage just bounces off of it.
General Franky is incredibly tough and possesses tremendous powers, such as air cannons, swords, and rocket launchers, as well as the ability of laser beams, which Franky ends up shooting himself after opening the cockpit. The General Franky is powerful enough to fight against an Ancient Zoan Devil Fruit user and that just goes to show how strong the General Franky transformation is.
5 Chopper - Monster Point
One of the most ghastly transformations seen in One Piece is Chopper's Monster Point. Chopper is a user of the Hito Hito no Mi and this Devil Fruit has turned his reindeer body into that of a human. He also has different forms, just like an average Zoan user does, however, when consuming the Rumble Ball, a drug that he created himself, he gains access to several other forms, which he calls points, that boost different abilities of his fruit.
While some boost jumping, others boost his muscular atrophy, and some others increase his focus and thinking capabilities. The most dangerous out of his transformations is Monster Point, which he first unleashed against Kumadori in Enies Lobby. This transformation sees him take the form of a giant capable of smashing through anything and everything in its path. Clearly, this is one of the strongest Straw Hat transformations in One Piece.
4 Zoro - Asura
Zoro is among the strongest members of the Straw Hat Pirates and he is a swordmaster worth 1.111 billion berries. He is exceptionally skilled when it comes to swordplay and his strong spirit plays a role there as well. When pushed to the limit, Zoro is able to bring a demon form out of his body, known as Asura. This Ausura transformation sees him develop two more bodies from within his body. It is as if Zoro has three heads, six arms, and six legs.
Instead of just three swords, he ends up wielding nine and this form sees him fight at his very best. His demonic aura has been felt by many across the many arcs and the truth about it has not yet been explained. Zoro was able to easily defeat Kaku with this transformation and even clash against Kaido momentarily. Zoro's Asura is among the strongest Straw Hat transformations in One Piece.
3 Nico Robin - Demonio Fleur
Nico Robin isn't one for unique transformations, however, One Piece's Wano Country arc saw her receive her very own unique transformation which is also true to her character. In her fight against Black Maria, Nico Robin ended up tapping into her dark side. Being called a Demon Child as a slight her whole life, Nico Robin embraced her identity to protect her friends and this gave rise to Demonio Fleur.
This transformation of hers sees her take the form of a giant demon with two sharp fangs and massive wings. When Robin takes this form, she gains immense strength, great enough to physically overpower an Ancient Zoan Devil Fruit user worth 457 million berries. Robin's transformation is among the very best in the Straw Hat Crew.
2 Sanji - Germa Mode (Ifrit Jambe)
Sanji is the cook of the Straw Hats as well as the man known to be one of the Wings of the Pirate King. His importance to Luffy and the group is immense in more ways than one. Sanji isn't simply a cook, he is also one of Luffy's strongest fighters, as well as an incredible strategist, and a leader. Sanji has been running from his past for his entire life and he returned to his family in the Whole Cake Island arc to face his trauma once and for all.
In the Wano arc that followed, Sanji's body went through some changes and he ended up awakening the latent abilities of Germa that he thought he did not possess. Amazingly, his emotions were spared, thanks to his mother, Sora, but his body turned into that of a monster capable of dismantling even a pirate worth 1.32 billion berries with rather ease. Sanji's eyebrows flip direction when he enters this transformation and he gains an incredibly tough exoskeleton, an extremely high recovery rate, more muscle mass, increased strength, and increased speed, as well as a major boost to all his other physical attributes. Above all, Sanji gains the ability to support hotter flames and lightning on his legs, which he manifests through Ifrit Jambe.
1 Luffy - Gear 5th
Among all the transformations in One Piece, none is as iconic as Luffy's Gear Fifth. Luffy ended up Awakening his Devil Fruit during the Wano country arc and he gained the powers of Nika, the Sun God. As a character, Luffy has always been free-spirited and childish at heart. His thoughts and intentions are mostly always pure and innocent and his Devil Fruit Awakening is the personification of this character trait of his at the very highest level.
Gear 5 sees him transform into the Warrior of Liberation. He fights with immense freedom, doing what he likes best, laughing all the way, and bringing smiles to the faces of the people. He frees people from their cages as well as spreads laughter and joy wherever he goes. Gear 5th is an unorthodox transformation when it comes to Shonen protagonists, however, it is the culmination of everything that Luffy has always been and, for that reason, it is the most iconic Straw Hat transformation in the story and up there with the greatest in fiction itself.