Ever since Eiichiro Oda first formally introduced it in One Piece, despite having subtly debuted at the very start of the story, Conqueror's Haki is a rare yet highly desired skill that permits an individual to manifest and exert their will on to others. This power can be used to incapacitate opponents, knock them out, or, in more advanced applications, coat a user's attacks to bolster their offensive power beyond imagination. Most of the strongest characters in One Piece are extremely proficient Conqueror's Haki users, including the likes of Shanks, Monkey D. Garp, Whitebeard, Kaido, Big Mom, Silvers Rayleigh, Kozuki Oden, and finally, the Pirate King himself, Gol D. Roger.While many of these names belong to the old guard of pirates who conquered the seas, there is a new group pf upstarts who are challenging the status quo and inspiring countless others to follow suit and take to the seas in search of Roger's legendary treasure.
This group, known as the Worst Generation, consists of 12 infamous pirates who rose to prominence between the Golden Age of Piracy and the New Age after the Summit War of Marineford. Besides Blackbeard, who was already in the New World at the time, 11 of these pirates, known as the "Supernovas," arrived at the Sabaody Archipelago, and entered the New World almost simultaneously. Since their introduction, many of these pirates have gone on to either ally with or face off against the Straw Hats, playing a major role in the narrative as it enters the Final Saga. Considering how they may affect the story to an even greater degree going forward, it is worth examining their ranks to see who among them is the most adept at using Conqueror's Haki in its many forms.
The Clear Winner
Having awakened his Conqueror's Haki shortly before the time skip, Luffy has been using this ability for the longest duration among the Worst Generation, as per the information the series has provided so far. During the two years he spent training with Rayleigh, the Straw Hats' captain refined his usage of the Color of the Supreme King to a level where he could trigger it at will, instead of the haphazard manner in which he used it during the Summit War of Marineford.
At the time of the Fishman Island Arc, Luffy's mastery of Conqueror's Haki was such that he could knock out tens of thousands of weaker opponents instantly, which is a testament to his progress. Prior to his final confrontation with Kaido, Luffy had also been training his advanced Armament Haki, known as Ryuo in Wano. Although this did help him in his fight with the Emperor, it was the realization that he could imbue his attacks with Conqueror's Haki that gave him the boost in power needed to fight on an even footing. This was later augmented by his Devil Fruit Awakening, which gave him the freedom to fight without any inhibitions, and score a decisive victory against Kaido.
Initially, his control over this ability was still quite unrefined, but he managed to improve upon it over the course of the battle, eventually gaining control of it to a degree that was nearly on par with Kaido himself. Hence, since, he has mastered the powers of domination, incapacitation, and now, is able to infuse Conqueror's Haki into his attacks, it is safe to say that Luffy is the most proficient among the Worst Generation in using this form of Haki.
The Next Best
Besides Luffy, the only other known Conqueror's Haki users among the Worst Generation is quite short, with only Roronoa Zoro and Eustass Kid having awakened this power. The revelation of Zoro being able to use Conqueror's Haki came quite late in the Wano Arc, with the fight against Big Mom and Kaido featuring the first instance of confirmed usage. At the time, Zoro himself was unaware that he could use this power, and only the statements of Big Mom and Kaido alerted him to the possibility that he could harness it. He was even able to inflict a deep wound on Kaido's near impenetrabe skin, something only Kozuki Oden had ever managed to do before.
The main reason behind why Zoro was able to unlock his Conqueror's Haki was his possession of Enma, the legendary sword of Kozuki Oden. Constantly draining his Haki with every swing, Enma drew out Zoro's dormant Conqueror's Haki during the fight against Kaido, and later against the Lunarian King. The Straw Hats' swordsman was able to quickly infuse it into his attacks after realizing that he had this ability in his arsenal, which elevated his swordsmanship beyond anything he was capable of before. In a way, Zoro's discovery of his Conqueror's Haki was expected by many, and it proved to be a decisive factor in his eventual victory against King.
Kid is said to have been able to use this power even at the start of the Wano Arc, according to Kaido's remarks, although he was still unable to match the Emperor's proficiency with it and was easily defeated. Moreover, he did not really show any real advancement with Conqueror's Haki usage even during the fight against Big Mom, relying more on his Devil Fruit Awakening to triumph over her when fighting alongside Trafalgar Law. On the otehr hand, when confronted by an advanced Conqueror's Haki user in Shanks, Kid was left entirely helpless and was defeated without putting up any fight, indicating that he has a ways to go before reaching the level of the Red Hair Pirates' captain. As the true extent of his capabilities when using Conqueror's Haki is still not clear, and he has not been shown using Conqueror's infusion, he falls below Zoro in terms of his Conqueror's Haki prowess.
The Potential Wildcards
As of the Egghead Island Arc at the start of the Final Saga, none of the other members of the Worst Generation are known to possess Conqueror's Haki in any form — be it the standard will projection abilities or more advanced applications such as infusion. While some may display such abilities later on in the story, the list can be shortened to Trafalgar Law and Blackbeard for the moment, since they have played the biggest part in the Straw Hats' journey so far. Trafalgar Law was shown to have Awakened his Devil Fruit, but did not display any hint of being able to use Conqueror's Haki even at the end of the Raid on Onigashima at the conclusion of the Wano Arc. Considering that he has been a major character ever since the Punk Hazard Arc, it might not be too much of a stretch for Law to awaken Conqueror's Haki at some point, but only time will tell.
The biggest question mark on this topic is Marshall D. Teach, also known as Blackbeard, who is the Admiral of the Blackbeard Pirates and one of the most powerful among the current crop of Yonko. He easily beat Law and his crew after their departure from Wano, which reveals a lot about the overwhelming power of the two Devil Fruits he has in his possession. Till date, there has been no instance or mention of him using Conqueror's Haki.
Despite this, since he is being built up as one of the final antagonists of the series, it would be nigh on impossible for him to survive in the New World without this power. The only thing that may work against him is his cowardly, scheming nature, which does not align with that of other Conqueror's Haki users in One Piece, as none of them would ever back down from a fight and possess all the qualities of a king. It will be quite interesting to see how this plays out in Blackbeard's case, as one does not simply walk into the New World and become an Emperor of the Sea without being able to use Conqueror's Haki — that is, unless Oda is going to reveal something more sinister that Teach has been hiding up his sleeve.