What is an antonym for which?

It depends on the context of how "which" is being used. Here are a few examples:

If "which" is used to ask for a choice:

* What: "Which color do you like?" vs. "What color do you like?"

* Who: "Which of you is going?" vs. "Who is going?"

* Whose: "Which book is yours?" vs. "Whose book is this?"

If "which" is used to refer to a specific item from a group:

* This/That: "I chose the red car, which was the fastest." vs. "I chose the red car, this was the fastest."

* Any/Some: "Which of these books is yours?" vs. "Any of these books could be mine."

If "which" is used to introduce a clause:

* That/This/Those: "I chose the red car, which was the fastest." vs. "I chose the red car, that was the fastest."

It's important to consider the specific situation to determine the best antonym.