Box calls are very easy, slates are pretty easy, and diaphrams are by far the most difficult of the 3 overall to use IMO most of the time. I use box calls to locate birds, slates occassionally when a diaphram isnt working, sometimes just changing a call does the trick. I mainly us diaphrams allot after I know where the birds are. Dan Thumma of Grand C Calls makes some great custom stuff. His 4 hole slate is about the easiest slate call Ive ever used.

http://www.grandccalls.com/Calls.htm