My absolute favorite is the mouth call. If you are good enough, there is not a sound that a turkey makes that can't be made on a mouth call. For most this comes with years of practice, but just a simple yelp can keep your hands free, and call in turkeys.
Slate, glass, or friction calls have there place. They are easy to run, and sound much like turkey.
Now for the meatstick turkey call for hunters from a beginner to a professional caller. The BOAT PADDLE box call. When all else fails, the boat paddle won't. These calls can be loud, but from a distance a trained ear can't distinguish the caller from a real turkey. If there were only one call, this would be it. My humble opinion.