The Cook of the Straw Hat Pirates, Sanji has gone from strength to strength over the course of the Straw Hat Pirates' journey across One Piece New world. He was always incredibly effective in battle, and now that the Straw Hats are facing serious dangers in the New World, especially in the final saga, he has had to strengthen himself even more.
In the final part of the story, the Straw Hat Pirates have risen to the level of an Imperial crew. This means that Luffy's minions, like Sanji and Zoro, have to reach the next level of strength as well. As such, Sanji has had to surpass many powerful pirates, and some elites have already been left in the dust by him.
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Vinsmoke Siblings
The Vinsmoke siblings are Sanji's very own brothers, and these characters were introduced to fans in the Whole Cake Island Arc. There it was revealed that the brothers were absolute war machines, capable of taking down countries by themselves.
The Vinsmoke siblings had everything Sanji didn't – an exoskeleton, a cold heart, and a body that could take down almost anyone. However, Sanji had one thing they didn't, and that was emotions and free will.
The Vinsmoke siblings were very strong, and by the time of the Whole Cake Island arc, some of them, like Niji and Ichiji, could have been stronger than Sanji. In Final Saga it is not like that. Sanji has officially surpassed every single one of them already, and he's reached unprecedented levels of strength ever since.
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Donquixote Doflamingo
Known for being the captain of the Donquixote Pirates, Doflamingo was a monstrously powerful figure that fans saw quite a bit of in the Dressrosa Saga of One Piece. Then Doflamingo was a rival for Luffy to take down.
However, Sanji had a fight of his own with him when the two met not too far from the Green Bit. Doflamingo was able to completely overpower Sanji then, even though he acknowledged his incredible strength. Sanji was much weaker than Doflamingo overall. But now, in the final saga, Sanji has become strong enough to easily defeat him, without even going all out.
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King
King the Wildfire, whose real name is Alber, was one of the Three All Stars of the Beast Pirates. Given that he had the highest bounty of them all, and the fact that the Beast Pirates are a meritocracy, King was considered the strongest underling by Kaido. He was incredibly tough, and his fight against Zoro was proof that King was not an easy opponent to face. Sanji clashed with King in battle on several occasions, and each time he was able to hold his own, suggesting that there wasn't much of a difference in power and skill between them to begin with.
However, once Sanji awoke and unleashed his latent powers, he reached a level far surpassing the Kings. Right now, Sanji has enough in him to easily defeat King in battle without even going all out.
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Charlotte Katakuri
Charlotte Katakuri is the strongest of the Sweet Commanders of the Big Mom Pirates. Katakuri is known to be an incredibly powerful pirate with the ability to see the future himself, and is above Sanji in the use of Observation Haki. But overall, as a fighter, Sanji far surpasses him in every aspect of combat.
After Sanji awakened his latent powers, he reached a level far above that of the Yonko First Commanders. Only a handful of individuals can take him on and beat him in battle, and Katakuri certainly isn't one of them. A battle between these two individuals would certainly be incredibly fun for fans to witness, as they both excel when using Observation Haki.
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Marco
Known for being the former First Division Commander of the Whitebeard Pirates, Marco is an extremely powerful pirate, and his strength has been recognized by even the Five Elders. In the Paramount War arc, Marco was shown to be strong enough to fight the Navy Admirals and give them a run for their money.
Marco is an incredibly powerful figure, and the power of his Phoenix Devil Fruit ensures that he can recover from any damage, as long as he can. While it's certainly debatable who is the strongest between Marco and Sanji, the latter has incredible toughness, along with a very high recovery rate himself. Also, the attack power that he possesses far surpasses anything that Marco has in his arsenal, and the fact that he was able to destroy even Kizaru's light and stun him, something that even Marco couldn't do, clearly suggests that Sanji is far more impressive than Marco.
One Piece is available to read via Manga Plus and Viz Media.
