In the expansive world of One Piece, Haki is one of the primary power systems, drastically increasing the user's senses and strength. Among the three types of Haki, Conqueror's Haki is widely regarded to be the rarest and most powerful.
Also called Supreme King Haki, this allows one to completely dominate others, causing targets to feel intimidated and even lose consciousness. More advanced forms let individuals infuse Conqueror's Haki into physical attacks or their weapons, causing widespread destruction.
Only the most talented and accomplished figures in One Piece have been capable of wielding it. While Sanji has not yet shown Conqueror's Haki in One Piece, most fans believe it is only a matter of time till he manifests it.

Unlike Observation Haki and Armament Haki, Conqueror's Haki cannot be learnt. It needs to be innately present within an individual and later unlocked. Within the Straw Hat Pirates, Luffy and Zoro possess it, displaying their prowess during desperate situations when the stakes are at their highest.
Sanji, one of the main fighters and the chef of Luffy's crew, has yet to showcase Conqueror's Haki. However, viewers are convinced that he will go on to unlock it, and that the time may be near in the manga. There are multiple reasons for this assumption.
Why One Piece fans are convinced Sanji will have Conqueror's Haki
The primary reason fans have always believed Sanji will possess Conqueror's Haki is simply because it would be unusual for him not to have it. Sanji is one of the main characters in One Piece, where his growing strength and development are always a highlight.
He has learnt both Observation and Armament Haki, and has a long-standing rivalry against Zoro. Additionally, Sanji confronts a primary antagonist in every arc. He has been able to snatch victory from the jaws of defeat till now. Still, in the Final Saga, where the antagonists are stronger than ever, the natural course of progression for Sanji's character will be to give him Conqueror's Haki.
This will allow him to stay on par with Zoro and continue being helpful to the Straw Hats’ struggle against the World Government and other villains. As mentioned, fans believe the time for Sanji to display Conqueror's Haki is not far off, especially with the introduction of Scopper Gaban, the Left Hand of the Pirate King.
Much like Luffy is compared to Gol D. Roger, and there are many similarities between Zoro and Rayleigh, the parallels between Sanji and Gaban are clear. When Gaban introduced himself as the Preacher of Love, Luffy even stated that Sanji would be like him when older.
Scopper Gaban is one of the strongest men in the world and is capable of using Conqueror's Haki. Therefore, fans believe he may become a mentor to Sanji, or that his presence may signify the passing of the torch to Sanji. Since this legendary figure has been greatly injured in the recent chapters of the manga, Sanji's time to shine may also be coming soon.
Conqueror's Haki is known to reside inside those who possess the qualities of a king. Sanji, with his selfless attitude and position as an heir to the Germa Kingdom, fits these criteria. Fans have even gone as far as theorizing what Sanji's Conqueror's Haki would be named.
Sanji's main attacks in One Piece are his fiery kicks. The first of these is the Diable Jambe, after which he added Armament Haki to create the Ifrit Jambe. Since both these names hold a connection to demons, the fandom is calling Sanji's final kick the Asmodeus Jambe. They anticipate this to be violet in color, with Asmodeus - the Demon King of Love, perfectly aligning with Sanji's personality.