1. What food do you like most?

Question

2. Which food do you llike the most?
3. What kind of food do you like best?
4. What kinds of food do you like the best?

(Are all the expressions grammatical?)

Answer 1

All are grammatical, and all are commonly used.

#2 is asking "which" of some specifically named foods you like most, but the others are inviting the listener to supply his/her own favorites.