Good question!
Everyone is different!
Some people just take to certain calls. I think that with a little practice the pot, and box calls are the best. I also think that the tube call is one of the most effective calls in the woods, bar none, its just a tone they dont hear very often!
PM me if you would like me to explain it more over the phone!
Good luck in your search and let us know what you choose!

Shoot Straight!
Willie