The Monster Trio form some of the strongest fighters of the Straw Hat Pirates in One Piece. They are the core of the Straw Hat Pirates when it comes to fighting and are incredibly powerful, each reliable in their own ways. The Monster Trio consists of Luffy, Zoro, and Sanji.
As the captain, Luffy is, obviously, the strongest in the entire crew, followed by Zoro, who is very closely followed by Sanji. The Monster Trio have put their mark on the series from the very beginning right to where the series is currently. With One Piece now in its Final Saga, the Monster Trio will be more important than ever before.
The Conception Of The Monster Trio
The idea of Monster Trio has been apparent in One Piece since the very first arc where these three characters fought together. In Arlong Park, all three of these characters combined for the very first time. Luffy took on Arlong, Zoro took on Hachi, while Sanji took on Kuroobi. All three of them won their respective fights and eventually, Arlong Park was destroyed and Cocoyashi Village was saved. From there onwards, it became a pattern for Luffy, Zoro, and Sanji to fight against the three strongest people in the enemy group.
For instance, in Arabasta, Luffy fought against Crocodile, Zoro fought against Mr. 1, while Sanji fought against Mr 2. Similarly, in Enies Lobby, Luffy fought against Lucci, Zoro fought against Kaku, while Sanji ended up fighting against Jabra. Once again, Oda established the order where Luffy stands at the top and he is followed by Zoro, who is very closely followed by Sanji. This also gives the fans an indication of how strong these fighters of the Straw Hat Pirates are. Luffy is certainly a level above Zoro while the difference between Sanji and Zoro isn't all that much.
Nonetheless, by the conclusion of Enies Lobby arc, the idea that Luffy, Zoro, and Sanji were the strongest members of the Straw Hat Pirates and, thus, monsters in their own right was already established. However, it wasn't until the Thriller Bark arc that the term Monster Trio was used officially for the very first time. It is a very big misconception among some fans of One Piece that the Monster Trio is a fan-made term. This is very far from the truth when, in reality, the Monster Trio is actually a canonical trio in the Straw Hat Pirates.
The Monster Trio was named by Oda himself in One Piece chapter 455, through Nami. When Oars was awakened by Moria, Nami, Chopper, and Usopp were hiding inside of Kumacy's body. That is precisely where she mentioned for the very first time that the Monster Trio of the Straw Hat Pirates were already taken down. This was the very first instance where this trio was canonically named by Oda. However, going down the line, this trio was mentioned again and again in the story.
Cementing The Monster Trio
Shortly after the the timeskip, the Straw Hat Pirates reunited on Sabaody Archipelago and after having their ship coated, they sailed for Fishman Island. While on their way to Fishman Island, Caribou made his way on the ship as well and he was inside a barrel, waiting for an opportunity to pounce. After being caught and sealed shut inside by Franky, Caribou witnessed Luffy, Zoro, and Sanji taking on Tsurume. Once again, Oda, through Caribou, referred to Luffy, Zoro, and Sanji as the Monster Trio who beat the kraken to half death.
Clearly, by this time, the concept of Monster Trio was already established by Oda. In the major fights in Fishman Island, fans could once again see Zoro and Sanji take the rear while Luffy confronted Hody. Moving forward in the story, the Straw Hat Pirates often split apart, and, because of that, fans did not get many opportunities to witness and the Monster Trio in action together, that is, until the story reached the Wano Country arc. Here, the Straw Hat Pirates once again relied heavily on the Monster Trio to defeat the crux of the Beast Pirates. Luffy took on Kaido, Zoro took on King, while Sanji took on Queen.
Furthermore, the events of Wano took things a step beyond and referred to Sanji and Zoro as the Wings of the Pirate King. This is, once again, a title that is specifically attributed to the members of the Monster Trio. Luffy, who is the head of the Monster Trio, is going to be the Pirate King while Zoro and Sanji are going to be his wings. It must be pointed out that the Wings always have to be equal. For Luffy to soar through the skies, no wing can be bigger than the other. This implies that Zoro and Sanji are equal in terms of authority and also extremely close in terms of power. Luffy requires both of them to be at a very high level in order for him to conquer the seas and become the Pirate King.
The Strength Of The Monster Trio
It goes without saying that the Monster Trio consists of the strongest members of the Straw Hat Pirates. Luffy, being the Captain, is surely the strongest and he's followed by Zoro, who is the master swordsman of the crew. Zoro is very closely followed by Sanji, who is the chef, and, at the very same time, also the strategist of the crew. These members are the powerhouses of the Straw Hat Pirates and while there are other powerful members of the crew as well, such as Jinbe, Robin, and Franky, Luffy, Zoro and Sanji are the core of this group when it comes to fighting.
When compared to the strongest fighters in the One Piece world, the Monster Trio are at the very top. Luffy, for instance, is already one of the Four Emperors of the Sea. This means that he ranks among the four strongest pirates and is strong enough to fight anyone alive in the world, no matter how strong they are. Zoro recently defeated King the Wildfire and as a number two of the crew, he is strong enough to contend with any other number two as well. The likes of Ben Beckman, among others, should all be in his league as well. Zoro is aiming for greatness, which will eventually take him to the level of Mihawk and beyond that, which means that he needs to aim very high.
Sanji, on the other hand, recently dismantled Queen with minimal effort. It wouldn't be a stretch to say that Sanji can even defeat stronger opponents, such as the Yonko First Commanders, with rather ease. This means that Sanji is also beyond the level of any First Commander and should be able to contend with them with the rather ease. By the end of the story, both Zoro and Sanji are going to be incredibly strong and close to the level of a Yonko themselves, which is indicative of the rivalry these to characters have and the portrayal that Oda has always shown them.
For now, the Monster Trio have already established themselve s as a superpower on the seas. While Luffy is already on the level of the Yonko and can even go beyond that, Zoro and Sanji are eventually going to get there. For now, they can defeat any Yonko Commander and contend with the strongest of pirates, however, there is still room for growth.
By the end of One Piece, the Monster Trio will have established itself as the strongest trio in the entire series, unmatched by any other. However, there is quite some time until One Piece finally reaches that point. For now, fans should sit back and enjoy their journey through Egghead Island, when the story returns this month.