Haki is one of the most important powers to exist in the One Piece world. This ability is present within everyone; however, there is a facet of this power that only a select few can utilize, called Conqueror's Haki. While Haki is not the primary power system in One Piece, it is absolutely essential if a pirate intends to conquer the latter half of the Grand Line, the New World.
The legendary pirates of the One Piece world have all held this skill and trained it to the highest level, which is how they made a name for themselves that has reverberated throughout history. The legendary pirates of One Piece have trained their Haki to the very best level, but not all of them have the same level of mastery over this skill.
Shiki, the Flying Pirate, is a legend who was once a rival to the likes of the Pirate King, Gol D. Roger. He was extremely powerful; however, he primarily relied on his Devil Fruit, the Paramecia-type Fuwa Fuwa no Mi. Fans have never seen him utilize Haki.
However, it can be assumed that he did have access to this skill. How far he had trained the skill and how strong it was remains unknown and, unfortunately, it is highly likely that fans will never find out.
Big Mom was one of the Emperors of the Sea until recently. However, now, her status remains unknown. Regardless, in her days as one of the Yonko, Big Mom was absolutely menacing. She was a master of her Devil Fruit powers, and while she could defeat the vast majority of people with her fruit alone, she also possessed tremendous control over Haki.
In her case, she was a user of all the Haki types. However, in particular, she was a very good user of Armament and Conqueror's Haki. Fans saw her use Conqueror's Haki Coating in Wano, using which she knocked out Page One in a single strike.
Known to be the strongest creature on land, air, and sea, Kaido was one of the Yonko of the Sea until recently. He was finally defeated by Luffy, but that doesn't take anything away from his strength. Kaido primarily utilized his Uo Uo no Mi, Model: Seiryuu in battle; however, he is one of the rare exceptions where both Devil Fruit power and Haki appeared to be on the same level.
Kaido trained his Haki tremendously to a point where he could knock out even the most seasoned pirates with a single blow of his Color of the Supreme King. Even after Luffy managed to learn this skill, Kaido displayed it at a level that was above that of the Straw Hat pirate. Overall, his Haki was extremely advanced. In fact, he could tap into the advanced forms of all three Haki types, being able to see the future, land attacks without making contact, and, at the same time, also coat himself in Conqueror's Haki.
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