When it comes to Haki, a select few stand above everyone else in the world of One Piece. In the Egghead arc, fans got to witness the strongest display of Haki ever, and it was done by none other than Joy Boy, the legendary pirate who lived long ago in the Void Century.
Joy Boy's Haki was quite easily one of the strongest, given that it was able to counter the powers of Imu's magic, as well as split the skies on its own. While Joy Boy's Haki remains the best in One Piece, there are a few other Haki users whose skill with it can rival even Joy Boy.
Rocks D. Xebec (Domi Reversi'd)
- Haki Types: Armament, Observation Conqueror's
Rocks D. Xebec was one of the strongest Haki users alive in One Piece. In his prime, he was known to be strong enough to easily kill an Admiral of the Marines. However, after Rocks was Domi Reversi'd, his powers reached a whole new level.
This form of Rocks D. Xebec received a significant boost to his Conqueror's Haki. His Haki was able to split the skies by itself, and fans even saw Roger and Garp struggle against him, mainly because his Haki was too strong to pierce. Eventually, Roger and Garp had to use their maximum power to pierce his Haki, which is supposed to show that his overall control over this was immense. Right now, Rocks's Haki remains the strongest that the fans have seen after Joy Boy, and is thus quite close to it in power.
Gol D. Roger (Prime)
- Haki Types: Armament, Observation Conqueror's
Gol D. Roger possesses one of the most impressive Haki that fans have seen in the entirety of One Piece. In the battle in God Valley that took place 38 years ago, Roger was able to pierce the defense of Rocks in his Domi Reversi'd form, but only with his maximum Haki output.
That said, one thing fans often forget is that this Roger was far from his prime. Over the next decade or so, Roger got even stronger and reached his prime level of strength. In his prime, Roger's Haki was most certainly stronger than that of Domi Reversed Rocks, even though fans have not seen it yet. At this point, Roger's Haki mastery would most certainly be close to the level of Joy Boy.
Shanks
- Haki Types: Armament, Observation Conqueror's
When it comes to mastery of Haki, few can ever even hope to come close to the level of red-haired Shanks. Shanks possesses monstrously powerful Haki, and in the entire series, he is the only one whose Haki has been directly compared to that of Joy Boy. Joy Boy's greatest Haki feat is being able to split the sky by itself, as well as knocking out Vice-Admirals of the Marines and cancelling the powers of Imu.
As far as Shanks goes, fans know that he can split the skies and even knock out Vice-Admirals, if Film Red is anything to go by. Most importantly, in the Egghead arc, Dorry and Brogy themselves considered Shanks' Haki close to the level of Joy Boy, being unsure as to which of the two was stronger. Undoubtedly, Shanks is the strongest Haki user among the pirates right now, and his Haki is close to the level of Joy Boy, if not equal to it.
Saint Imu Nerona
- Haki Types: Armament, Observation Conqueror's
Saint Imu Nerona possesses tremendously powerful Haki. In fact, their Haki is so strong that 38 years ago in God Valley, the combined attack of Roger, Garp, Whitebeard, Big Mom, and Kaido, along with Xebec, did not even scratch them because their Haki was so much stronger. The only Haki that has countered the powers of Imu is that of Joy Boy.
Even on Elbaf, it appeared that Imu's Haki was so strong that even Luffy started sweating. Hardly anyone in One Piece can come close to the level of Saint Imu Narona when it comes to Haki, and Oda will most certainly demonstrate its full powers later down the line in the Final Saga.
Monkey D. Luffy (Future)
- Haki Types: Armament, Observation Conqueror's
Monkey D. Luffy is a prodigy when it comes to Haki usage. Even though he possesses the power of the Nika Fruit, his Haki is equally as refined. Luffy possesses all three types of Haki, and he has advanced his Conqueror's Haki tremendously.
It would be inaccurate to say that Luffy has Haki on Joy Boy's level right now. However, he has the potential to get there, and he will most certainly even go on to surpass Joy Boy's Haki. Luffy's Haki continues to grow with each passing arc, and in Elbaf, he will learn to bypass immortality. Later down the line, Luffy's Haki will reach a level where he will even be able to surpass the strongest alive, such as Shanks, Roger, and eventually, Joy Boy.