This question comes up every year.
At one point in my life, I would have said my Quaker Boy Grand Old Master box call.
Probably my Quaker Boy easy yelper push pin call would have been an equally good choice, but. . . well us "experienced" callers aren't supposed to like push pin calls.
My home made shamanic dream series slate over glass pot call comes close.
However, I have to say that my Heirloom Double Barrel is now the answer. The reason for the change is simple: It has incredible volume and it has slate on one side and glass on the other. Here's a pic:
That's the same call-- it's a double sided pot call.