Kaido regards Haki as stronger than Devil Fruits, based on his interactions with formidable Devil Fruit users.

In the dynamic and action-packed universe of “One Piece,” the question of power takes center stage, with epic battles always captivating fans. In this world, two prominent sources of power stand out: Devil Fruits and the various forms of Haki.
Consumers need only consume one Devil Fruit to access the power it bestows, but each individual can only consume one fruit. Conversely, with Haki, everyone can access this power, but only a select few can truly master it.
So, which is stronger, Devil Fruits or Haki?
Kaido - one of the strongest characters in One Piece - holds a clear stance on the hierarchy of these abilities. The former Yonko firmly believes that Haki surpasses the abilities bestowed by Devil Fruits. His perspective is not merely a matter of personal opinion but is also evaluated based on his extensive experience and knowledge, gained through countless battles and encounters with powerful users of both abilities.
The Grand Line - a central backdrop in One Piece teeming with Devil Fruit users, each showcasing extraordinary abilities surpassing the limits of ordinary humans. Notable examples include characters like Whitebeard, capable of causing devastating earthquakes, Big Mom with her soul-manipulating power, Luffy wielding the rubber-making Devil Fruit power, and Kaido transforming into a massive, powerful dragon.

However, despite these impressive examples, Kaido still believes that Haki surpasses Devil Fruit abilities. This declaration is reinforced by legendary figures like Gol D. Roger and Kozuki Oden, who, despite lacking Devil Fruit powers, remain formidable and respected thanks to their mastery of Haki.
The ability to use Haki is innate in every individual, similar to the concept of Nen in “Hunter X Hunter,” but very few fully master it. Haki is divided into various forms and variants, each with its own characteristics and challenges.
Characters like Shanks, Monkey D. Garp, and Dracule Mihawk serve as evidence of Haki's potential, reaching the pinnacle of ability without relying on Devil Fruit powers. Their deeds and reputations testify to the raw, pure strength of Haki. The new generation, represented by characters like Zoro and Coby, also demonstrate the evolving potential for power in the world of One Piece.

Although Luffy, with the awakening of his Devil Fruit, managed to defeat Kaido, Kaido's claim of Haki's supreme power reflects a deeper truth. Greatness in One Piece is not merely defined by physical strength but by the amalgamation of physical prowess, strength of will, and profound understanding of one's own spirit.
In this sense, Haki represents not just power but also a journey of self-discovery and self-perfection accessible to all, yet only mastered by those with the most dedicated and resilient spirits.