“Fastball, because it sets everything else up.” Whether it’s your out pitch or not. that would be my reply.
Now if I’m trying to answer that like an interview question, I might say… “I don’t really have a best pitch, they all work together, my fastball sets up my other pitches and get’s me a strike when I need it. So if you need a specific pitch, I’d say my fastball. But I am confident I can use any pitch to get an out as long as I have more than one working that day.”