Devil Fruits are considered the treasures of the sea, and they are a representation of human evolution. Each Devil Fruit grants a unique power that elevates the overall strength of an individual. For a long time, fans believed that eating a powerful devil fruit was the only way to attain an ungodly amount of power.
But, there were proven wrong by Kaido, who reaffirmed that Haki is the strongest power in One Piece. There are three different types of Haki: Kenbunshoku, Busoshoku, and Haoshoku Haki. Out of the three, Kenbunshoku Haki is the more common type of Haki. Normally it can be used to protect oneself by avoiding attacks, but certain people who have trained a lot can actually use it to look into the future. This particular ability is called Future Sight. Due to the absurd power that it grants, only a handful of One Piece characters have managed to awaken it.
4 Shanks
Shanks is arguably the strongest character in the series. He is the chief of the Red-Hair Pirates, and under his guidance, they have become one of the most powerful crews in the world. He boasts of a bounty of 4,038,900,000 berries, which is currently the highest active bounty in the series. Before starting his crew, Shanks was a member of the Roger Pirates. He was influenced by Roger's way of thinking, and he adored him very much.
Like his late captain, Shanks doesn't have a Devil Fruit. Instead, he relies completely on his mastery over Haki. Shanks' control of Haki is on a different level from most characters. He can use all three types of Haki to the highest degree. While Shanks is known for his control over Haoshoku Haki, his Kenbunshoku Haki is not bad either. In the fight against Eustass Kid, Shanks used his Kenbunshoku Haki to see the future, after which he immediately defeated Kid with Divine Departure. Shanks also has a big advantage over other users of Future Sight, as he can prevent others from using the ability with his Haoshoku Haki. Given his strength, it's no wonder why Shanks is so highly respected by pirates and Marines alike.
3 Monkey D. Luffy
Monkey D. Luffy is the main character of the series. He is the captain of the Straw Hat Pirates and one of the Yonko. Luffy has fought many strong characters on his journey, and those fights have helped him mature as a fighter. Luffy was completely reliant on his Devil Fruit early on in the series, but he has changed a lot since.
Luffy learned how to use all three types of Haki while training under Silvers Rayleigh. However, it wasn't until he reached Whole Cake Island that Luffy started using the advanced forms of Haki. While fighting Charlotte Katakuri, Luffy was able to awaken Future Sight. It allowed the hero to take down the strongest Sweet Commander. Luffy's control over Future Sight improved further in the Wano Country arc. He used Future Sight to avoid many critical blows from Kaido, and it paved the way for his eventual victory over the Yonko.
2 Charlotte Katakuri
Charlotte Katakuri is one of the Sweet Commanders of the Big Mom Pirates. Katakuri boasted an unbeaten record in fighting, and even his siblings were quite proud of it. They believed that there was no one capable of defeating him in a fight. Katakuri ate the Mochi Mochi no Mi, which is a special Paramecia-type Devil Fruit. It allowed him to turn his body into mochi and create all kinds of structures with it. Katakuri's Devil Fruit control was far superior to most of the characters in the series.
But, his mastery over Haki wasn't all that bad either. Katakuri could use all three kinds of Haki, and he was the first individual to use the advanced form of Kenbunshoku Haki. With its help, Katakuri was able to look into the future, which gave him a distinct advantage over his opponents. While using Future Sight, Katakuri nearly pummeled Luffy to death, with the Straw Hat only surviving after he awakened the ability himself. The combination of Future Sight and Devil Fruit made Katakuri a fearsome opponent.
1 Kaido
Kaido was known as the "strongest creature in the world." There were very few characters in the series who could injure him, never mind defeat him. Kaido was exposed to wars and fighting at a very young age. He became famous for his strength, but the people that he fought for tried to sell him to the World Government. After running away, Kaido eventually founded his pirate crew. Kaido possessed the Uo Uo no Mi, Model: Seiryu, a Mythical Zoan-type Devil Fruit that allowed him to turn into an azure dragon.
In his dragon form, Kaido covered almost the entire sky above Kuri. Even after becoming a Yonko, Kaido's desire for battle remained insatiable. He kept fighting strong opponents, and it reached the point where no one could match him. As an individual whose mind was solely occupied by the desire to become strong and wage wars, it wasn't surprising to see Kaido having the advanced versions of all three types of Haki. Kaido used Future Sight in his fight against Luffy, but only for a short period of time. He didn't utilize it too much because he was confident that his indestructible body could withstand all kinds of attacks.