The slate (or pot) call as the easiest to learn to use well enough to kill turkeys. Mastering one takes practice.
There's a bunch of different surfaces out there. A good one to begin with is real slate or one of the ceramic-type surfaces (like K & H's "sla-tek" for example). Another surface that's good is crystal. The Drurys have a "power crystal" that works very very well.
Don't worry about calls made from exotic hardwoods and surfaces. They're just as likely to not work as work, it changes from one day to the next in the turkey woods. Many turkeys are and will be killed with off the shelf calls.
And, one final piece of advice. When you find one that works well for you, buy 2-3 for reserve. Call companies have an annoying habit of discontinuing calls and releasing "new & improved" versions that aren't alwyas improved.