As the series' ending draws nearer, Luffy is now arguably among the strongest characters in the world thanks to Advanced Conquerors Haki, arguably equaling Kaido in battle. We say arguably equaling Kaido because Kaido is facing Luffy while lifting an entire island with his power, after defeating multiple enemies who managed to hurt him, while already defeating Luffy before only for Luffy to recover thanks to the help from other people... and now Luffy can battle Kaido equally.
The point is, Advanced Conquerors Haki has a lot to do with Luffy being strong enough to fight a Yonko. It is very safe to assume that the strongest characters in One Piece have the same ability. And while not many are confirmed to have it right now, it is only a matter of time until these monsters in this list are revealed to be masters of it.
Silvers Rayleigh is the most skilled all-around master of Haki we have seen so far. One of the conditions of Advanced Conquerors Haki seems to be the mastery of Advance Armament Haki, and Rayleigh is an absolute boss in this area.
His introduction is him casually taking the slavery bomb collar off of Camie's neck; a feat only possible for users of Advanced Armament Haki. Then with just a raise of a hand, he can bring a giant elephant to unconsciousness, an elephant that Luffy would have a hard time defeating in his pre-Haki days.
Furthermore, he is the vice-captain of the Rogers Pirates. If Gol D. Roger has Advanced Conquerors Haki, and Oden has learned Advanced Conquerors Haki during his time with the Rogers Pirates, then it would be a safe bet to say Rayleigh has it too.
Dracule Mihawk's entry on this list has everything to do with a certain Straw Hat Swordsman. It would be very embarrassing for Mihawk when he and Zoro finally clash and he is defeated right away because his legendary black sword gets broken by the impact of Zoro's Haki.
It is just common sense to assume that if he is the world's strongest swordsman, and Zoro has not reached his level yet, then any special skill Zoro has learned with the sword, Mihawk can counter or do it too.
Another factor is that he is also a rival of the Yonko Red-Haired Shanks. Shanks after all is featured as one of the people that can do battle with Kaido, and the prerequisite to do that is with Advanced Conquerors Haki.
Mihawk and Shanks cannot regularly clash in the past if one of them does not have Advanced Conquerors Haki.
The very premise of why Luffy and Shanks are still not meeting is because the promise that Luffy made to Shanks, is for him to grow up to be just as great or better than Shanks. If Luffy has Advanced Conquerors Haki right now, and he and Shanks are still not meeting, then Shanks must have Advanced Conquerors Haki.
Red-Haired Shanks's face is also seen when Kaido is remembering the few people that can fight with him on equal ground. Since Advanced Conquerors Haki is a prerequisite to harm Kaido in any meaningful way, then Shanks must have it.
Also, another character that obviously has Advanced Conquerors Haki is Dracule Mihawk. They cannot be rival swordsmen if only one of them has Advanced Conquerors Haki, wouldn't they? Shanks sword would immediately get broken that way.
Monkey D. Dragon
As of right now in the middle of the Wano Arc, Monkey D. Dragon still does not have a big part in the story. When the time does come when he and his son Luffy would meet, he needs to have a credible presence as a mentor figure or a threat, and Dragon cannot do that if he does not have Advanced Conquerors Haki.
At this point, if a character has a reputation as one of the most dangerous characters in One Piece, then he must have Advanced Conquerors Haki. Otherwise, why would we take him seriously? Dragon is after all this most wanted man in the world.
Rocks D. Xebec
Rocks D. Xebec might be the most powerful character in the One Piece universe. Not only was his crew consisted of all-star pirates with three of them future Yonkos, but it took the combined might of Gol D. Roger and Monkey D. Garp just to take him down.
With that kind of battle prowess, it would be harder to believe he does not have Advanced Conquerors Haki. The pirates that worked under him; Kaido, Big Mom, and Whitebeard, are all confirmed Advanced Conquerors Haki Users, yet he isn’t? Not to mention Gol D. Roger. At this point, it would be impossible if he does not have it.
Monkey D. Garp
As far as we can tell, Garp does not have any Devil Fruit powers and just get by with his awesome strength and mastery in Haki. Although, "getting by" may not be the word when we are talking about the most celebrated Marine officer of all time and Gol D. Roger's rival in their prime.
That is to say, if Gol D. Roger had Advanced Conquerors Haki, then logic dictates that Garp must have it too for them to be considered equals and rivals on the battlefield.
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