Aside from Devil Fruits, Haki is one of the most important powers wielded by characters in the One Piece world. There are three forms: Haoshoku Haki, also known as the Conqueror's Haki; Kenbunshoku Haki, also known as Observation Haki; and Busoshoku Haki, also known as Armament Haki. All three allow the user to harness their spiritual energy to accomplish a wide range of tasks. Few people can use all three, in part because all are difficult to use, and in part because only a few million people are born with the capacity to learn Haoshoku Haki.
Because it's so difficult, it makes sense that strong Haki users are among the strongest characters in the series. From Straw Hat pirates like Luffy and Zoro to legends like Gol D. Roger and Whitebeard, these characters can pull off incredible moves using Haki. Are they truly the best? That's up to you to decide.