Question

(which, what) is the most difficult subject for you?

Ans - Which

Answer 1

It all depends on context. What is the whole paragraph?

Answer 2

There is none. It is a fill in the blank with the correct interrogative pronoun ( Who, Which, whose, whom, what)

Answer 3

"Which" is correct if it's understood that the choice is between two or more specific subjects.

Example: Which is your most difficult subject this year?

"What" is correct if the specific subjects aren't clear.

Example: What is your most difficult subject?