Down below, I have listed the different types of Haki and level of techniques that Luffy, Zoro, and Sanji can use, and the type of Haki that Oda officially lists is their specialty.

Luffy

Conqueror’s Haki: Advanced Level Techniques.

Observation Haki: Advanced Level Techniques.

Armament Haki: Advanced Level Techniques.

Zoro

Conqueror’s Haki: Advanced Level Techniques.

Observation Haki: Basic Level Techniques.

Armament Haki: Basic Level Techniques.

Sanji

Observation Haki: Advanced Level Techniques

Armament Haki: Basic Level Techniques

Luffy

Currently, Luffy has shown to be able to use all of the advanced level techniques of each type of Haki, and I believe that this trend will continue in that if there are any more different Haki techniques introduced, Luffy will end up being able to use them.

Zoro

Given that Armament Haki is Zoro’s specialty, I believe that he will end up being able to use Advanced Conqueror’s Haki and Advanced Armament Haki that have been and might be introduced in the series.

Sanji

While I know that there are some people who are split on whether or not Sanji will end up having Conqueror’s Haki, I don’t think he will indeed end up having it. I think Sanji becoming stronger through his flames and now genetic Germa powers would be better than Conqueror’s Haki for him. Sanji will excel in terms of speed to be one of the fastest characters in the series.

Also, since Observation Haki is his specialty, I predict that he will gain the ability to use the Future Vision technique and any other advanced Observation Haki techniques that may be revealed in the future.