Kaido, the strongest creature on earth, arguably the one who has the strongest Mythical Zoan power himself stated that Haki is superior to Devil Fruit.
Haki, being as essential power in the One Piece world, usually raises better fighters than Devil Fruits. Being a spiritual energy, learning to use this power trains the body and spirit, and senses of the user, making them stronger.
It also allows Haki users to strike Devil Fruit eaters they otherwise couldn't, including Logia-type users. Alongside Sea Prism Stone, Armament Haki is one of the few ways to counteract the most powerful Devil Fruits.
Kenbunshoku Haki, one of the three Haki types in One Piece, grants the most powerful pirates unfathomable abilities.
He is a user of the Ope Ope no Mi, the ultimate Devil Fruit. During his fight against Vergo, Law was able to slice through his body, bypassing his Haki in the process, despite not using any on his own blade. In doing so, Law was able to beat Haki with pure Devil Fruit power.
Thankfully, Logia users' intangible bodies can be damaged with Armament Haki. However, if Haki didn't exist, the Logia types would be almost invincible in battle. While they can be countered by other elements, some, such as Akainu's Magu Magu no Mi, are simply too powerful.
Conqueror's Haki is one of the rarest powers to exist in the One Piece world. Unlike the other two Haki forms, this power isn't accessible by everyone but exists within only a chosen few. Its basic level allows the user to intimidate and overwhelm those who have weaker wills.
In fact, Nami has already demonstrated abilities similar to Haki. During the Alabasta Saga, she has shown the ability to tell even the slightest of changes in weather which, according to Vivi, is beyond human comprehension.
Luffy has mastered all three types of haki, including Kenbunshoku Haki (Observation Haki), Busoshoku Haki (Armament Haki), and Haoshoku Haki (Conqueror's Haki). He can even use several subforms of the hakis, which makes him one of the most powerful haki users in the One Piece world.
A glimpse of Shanks' power reveals that he holds the title of "The Killer of Observation Haki," and here's why. Shanks is one of the strongest characters in Eiichiro Oda's One Piece.
Luffy uses Haki unintentionally just as Ace is about to be executed. Luffy made a way through the execution platform with the help of Whitebeard Pirates and Allies, but Garp intervenes before Luffy can make it to the platform.
4 Charlotte Katakuri
Charlotte Katakuri is one of the Sweet Commanders of the Big Mom Pirates. Before fighting Luffy, Katakuri boasted of an unbeaten record. Katakuri can use all three types of Haki and he can actually use the Advanced versions of both Armament and Observational Haki.
He is proficient in both Armament and Observation Haki, as well as one of the few known characters who possess Conquerors Haki.
While this is shown to be a very rare gift among the world as a whole, it is noted by Don Chinjao that the New World is filled with numerous others who possess the ability to use this Haki. He explains that Luffy will most certainly encounter them and fight them for the title of Pirate King.
Sanji is an extremely skilled Haki user. During the two years he spent at Maiden Island, he trained his usage of both Armament and Observation Haki. Unlike Zoro and Luffy, Sanji wasn't taught this skill by anyone. He managed to learn all by himself and that is certainly an incredible feat.
Luffy develops another technique, Fourth Gear (ギア 4 ( フォース ) , Gia Fōsu) (similar to Third Gear), inflating his muscles with air to increase the size of his body (except for his legs) while his limbs are coated in Armament Haki.
Appearance. Zoro is a tall, muscular young man with lightly tanned skin and black eyes. He always carries his three swords along with him, bundled up with a green haramaki over his right hip, allowing him to easily draw them with his left hand.
In the anime, the user of Haoshoku Haki has a dark purple aura around him.
The Armament haki (busoshoku) is the ability to project your spirit into armor. An example of how it represents one's spirit is Vergo's purple haki as per debut. Vergo maybe mastered his armament haki. So this purple tint to his haki could've been his spirit's nature, the same to Luffy where he got his own shade.
He is a specialist of Observation Haki, Ryuo, and also Conqueror's Haki. Among all the Haki types, however, Luffy is especially skilled at using Conqueror's Haki, which is his best Haki type.
His sickness prevented him from using it at will. Sometimes he could like when Shanks came on the boat and they clashed other times he would start coughing up blood as he did in Marineford.