Seeing this many people answer " frictions calls" makes me very happy that I rarely use one when hunting.

I' ll take a box call over any of them. I would say mouth call as it is my favorite, but you asked for " most productive" . With practice, there isn' t a single call that can' t be done on a double-sided box, including a gobble. Plus, they have probably fooled more turkey (and other hunters) in the woods than any other call.