Monkey D. Luffy is the captain of the Straw Hat Pirates and one of the Four Emperors of the Sea. From a young age, Luffy has been clear that his desire is to become the King of the Pirates. However, such a dream would require a great deal of effort, and Luffy knew this, which is why he is always ready to fight.
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So far, Luffy has beaten many powerful opponents, and a lot of them were members of the Warlords of the Sea. Luffy didn't have an easy time facing them. In fact, he had to dig deep and rely on support from his crewmates. Another thing to remember is that while he fought a lot of Warlords, not all of them were similar in strength.
This post contains spoilers for One Piece.
Buggy
Captain Of The Buggy Pirates
Buggy is the captain of the Buggy Pirates and the current leader of the Cross Guild. The first time Luffy fought Buggy, the latter was not a Warlord. He was only an infamous pirate from East Blue. Luffy had to fight Buggy because he threatened the peace of Orange Town.
Despite having a Devil Fruit, Buggy stood absolutely no chance against his opponent. In fact, if Luffy hadn't been restrained early in the arc, there is every possibility that he would have beaten Buggy within minutes. Unlike most of Luffy's victims, Buggy eventually found success through his falsified reputation. As the world learned about Buggy's connection to the Roger Pirates and Shanks, he was named a Warlord and, after Kaido and Big Mom's fall, an Emperor of the Sea.
Currently, Buggy is also searching for the One Piece, which increases the possibility of him crossing paths with Luffy yet again. Unfortunately, Luffy is leagues above Buggy, so the former should once again beat him without any problems.
Gecko Moria
Led The Thriller Bark Pirates
Gecko Moria was another Warlord who was defeated by Luffy. Many years prior to the current timeline, Moria was a strong pirate, capable of clashing with Kaido. However, his mindset changed after losing his entire crew. Instead of recruiting new people, Moria began creating zombies that would never die. Furthermore, he became overly reliant on Devil Fruit, which is why Luffy was able to beat him. Even then, Luffy needed the temporary "Nightmare" power-up to fight Gecko Moria.
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Moria vs. Luffy was a representation of how a person's willpower can change everything. Moria had been lazing around for years and had lost interest in being a pirate. Meanwhile, Luffy loved challenges, which is why he was always ready to fight, no matter who his opponent was. After losing to Luffy, Moria's life crumbled, and the World Government even ordered his assassination. He was lucky to survive but had to run away.
Donquixote Doflamingo
Leader Of The Donquixote Family
Luffy fought Doflamingo, as he was the main villain of the Dressrosa arc. Doflamingo spent years terrorizing the people of Dressrosa, as no one had the strength to bring him down. Thankfully for the citizens of Dressrosa, Luffy came to Dressrosa to crush Doflamingo. However, the latter wasn't going to go down without a fight.
Doflamingo had access to the Ito Ito no Mi, a Paramecia-type Devil Fruit that allowed him to create and control strings at will. The strings that Domain created weren't fragile either. They could easily slice through a massive meteor, so cutting human beings was a non-issue. Doflamingo also had access to all three types of Haki, which only added to his ferocity as a fighter.
With so many outstanding abilities, it's hardly surprising that Luffy was forced to use Gear 4th against Doflamingo. If the hero hadn't trained with Rayleigh, there is no doubt that Doflamingo would have emerged as the victor.
Crocodile
Founder Of Baroque Works
Crocodile was the first Warlord that Luffy ever fought in One Piece, and he served as the primary antagonist of the Alabasta arc. Luffy had to fight Crocodile because he was trying to destroy Alabasta, Vivi's country. In the arc, Luffy fought Crocodile thrice, and surprisingly, the hero lost his two encounters. Crocodile left him on the cusp of death twice, but the protagonist survived. Finally, the third time, Luffy understood Crocodile's weaknesses and ended up winning.
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Despite being introduced so early, Crocodile was far from a weakling. He has the power of the Suna Suna no Mi, with which he can manipulate sand at will. This particular ability was especially troublesome in an arid country like Alabasta, which received little to no rain. Aside from creating sand, Crocodile can also drain the moisture from the bodies of his enemies, leaving them severely dehydrated. Presently, Crocodile serves as the co-founder of the Cross Guild alongside Dracule Mihawk.
Dracule Mihawk
Strongest Swordsman In The World
Dracule Mihawk is the strongest swordsman in the world, meaning there is absolutely no one who is better at wielding a sword. Mihawk garnered a fearsome reputation for himself by hunting down Marines, which ultimately forced the World Government to offer him a post among the Seven Warlords. Mihawk wields Yoru, an all-black blade that belongs to the Saijo O-Wazamono grade.
In addition to his swordsmanship, Mihawk also excels at using Haki. Presently, he is confirmed to have Armament and Observation Haki, but he likely possesses Conqueror's Haki as well. Luffy and Mihawk have not engaged in a proper 1v1 duel at all, and chances are they never will, but they did have a short tussle in Marineford. Luffy knew he was outclassed and tried to run away. However, if the current version of Luffy were to fight Mihawk, then things would be different.
Blackbeard
Captain Of The Blackbeard Pirates
Marshall D. Teach, also known as Blackbeard, is the captain of the Blackbeard Pirates and one of the Four Emperors of the Sea. Since Blackbeard became an Emperor in such a short time, it can be easy to forget that he was also a Warlord. He met Luffy in Mock Town, but the two didn't engage in combat until the Impel Down arc. Obviously, Blackbeard was leagues above Luffy, so he ended the fight quickly.
A lot has happened since, and currently, both Luffy and Blackbeard are extremely overpowered. The latter in particular possesses unbelievable strength. He is the only person in One Piece with two Devil Fruits, and both are dangerous. Furthermore, Blackbeard can also utilize two types of Haki. Whether he possesses Conqueror's Haki is unknown, but Oda will certainly have planned it.
