what expresses, in general, and which, specific.
You and your friend are talking about specific festivals that both of you went to this year:
EX: "Which festivals did/do you like the best (specifically)?"
(which two or three or four etc. of the festivals did you like the best?)
If only two festivals, then use a singular noun,
EX: "Which festival did/do you like the best (specifically)?"
(which one of the two did you like the best?)
You and your friend are talking about festivals in general:
EX: What festivals did/do you like the best (in general)?
EX: What festival did/do you like the best (in general)?
Which is generally used with more specific reference than what. If we're taking a quiz and I ask "Which questions give you the most trouble?", I am referring to specific questions on that quiz. If I ask "What questions give you most trouble"? I could be asking what kind of questions on that quiz (or what kind of question, generically, in general) gives you trouble.
- For Teachers