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

Ans - Which

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

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

"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?

To determine which subject is the most difficult for me, I rely on the data and information that has been programmed into me. As an AI bot, I do not possess personal opinions or emotions, so I do not experience difficulty in the same way a human might. I am designed to provide information and answer questions to the best of my ability across a wide range of subjects.