Monkey D. Garp is nothing short of a living legend. He is known as the "Hero of the Marines," for his contribution to the organization. Throughout the series, many characters have openly acknowledged Garp's strength. Garp is one of the few One Piece characters who can use all three types of Haki. He demonstrated his mastery over Haki in the Egghead arc.
Garp's achievements include defeating the Rocks Pirates, capturing Shiki, and destroying a large part of Hachinosu. Furthermore, in his heyday, Garp was able to compete against Gol D. Roger. There is no doubt that Garp is among the strongest One Piece characters, but there are certain characters who can defeat him.
6 Blackbeard
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Blackbeard is the captain of the Blackbeard Pirates and one of the Yonko. He is the only character who has eaten two Devil Fruits. Blackbeard ate the Yami Yami no Mi after stealing it from Thatch. The Yami Yami no Mi can cancel the power of other Devil Fruits, which is easily one of the most overpowered abilities.
Blackbeard also possesses the Gura Gura no Mi, a Paramecia-type Devil Fruit that allows him to create quakes at will. Blackbeard is able to use Busoshoku and Kenbunshoku Haki, which only adds to his freakish power. Given his impressive arsenal, it is not surprising that he is a strong candidate to take down Garp.
5 Shanks
Shanks is an extremely popular character, which is surprising considering his limited screen time. He has only made a handful of appearances in the story, but they have proven enough to give the fans an understanding of his strength. Shanks is easily the best Haoshoku Haki user in the series.
He can use it to nullify the Future Sight of other characters, giving him an advantage in battle. Shanks has also utilized Haoshoku Haki to coat his attacks, thus increasing their damage output even further. Shanks' burst of Haoshoku Haki damaged Whitebeard's Moby Dick and knocked out several members of the latter's crew. Garp bears hatred toward Shanks because the red-haired Yonko was responsible for motivating Luffy to become a pirate, so a fight between them wouldn't be unlikely.
4 Kaido
Kaido was the main antagonist of the Wano Country arc. He came to the country many years prior to the start of the story and took control by force. Kaido ruled Wano until Luffy and Co.'s arrival. Kaido was known as the "Strongest Creature in the World," and he showed why he was deserving of the title.
Kaido clashed with Luffy and other Supernovas at the same time. He completely obliterated Eustass Kid and took him in as a prisoner. Kaido could also use all three types of Haki; his Haoshoku Haki in particular was extremely refined. Kaido could also use Future Sight to defend against enemy attacks, but he rarely used it as his body was nearly indestructible. Bearing in mind his feats, Kaido would have proven to be a tough opponent for Monkey D. Garp.
3 Gol D. Roger
It is impossible to have a discussion about the strongest characters in One Piece without including the Pirate King himself. Gol D. Roger turned the entire world on its head with his might. Roger was the only pirate in the world who managed to conquer the Grand Line.
It is obvious that he couldn't have done it if he didn't have enough power. Roger fought against many notable characters in his lifetime, such as Shiki, Whitebeard, Xebec, and Garp. Roger and Garp fought each other many times, and the two had multiple close death encounters. It is unclear who had the upper hand, but since Garp never captured Roger, it is plausible to assume that the pirate was superior to the Marine.
2 Rocks D. Xebec
Rocks D. Xebec was the captain of the Rocks Pirates. According to Sengoku, Rocks was Roger's greatest rival, which is a big testament to his strength. Rocks' combative abilities are relatively unknown, but one can make an educated guess by piecing together bits of information that have been revealed so far.
Rocks was named by Kaido among the few characters he considered capable of defeating him. He was also able to keep a crew of Big Mom, Kaido, Whitebeard, Shiki, Ochoku, etc. together. Rocks was considered the biggest threat to the authority of the World Government, and it took the combined efforts of the Marines and Roger Pirates to vanquish him.
1 Whitebeard
Whitebeard was a legendary pirate who used to be a member of the Rocks Pirates. After the crew's defeat at God Valley, Whitebeard went on to start his own crew, and it didn't take long for him to make a name for himself. Due to his Gura Gura no Mi, Whitebeard was said to have the power to destroy the entire world. In addition to his Devil Fruit, Whitebeard could also use all types of Haki.
Buggy, who was an apprentice on Roger's crew, said that Whitebeard was the only person to ever fight toe-to-toe with his captain. This was further proven in a flashback when the Roger Pirates fought the Whitebeard Pirates for several days. In the flashback, Oden, who was able to overpower Kaido, was sent flying by Roger. Whitebeard, on the other hand, easily equaled Roger's power. Even Garp called Whitebeard "The King of Seas." So, it is beyond any shadow of a doubt that Whitebeard could have defeated Garp in a fight to the death.