Ultimately, the coach that gets the most out of your Athlete is the one to stay with....My daughter is only 12U and has been multiple pitching coaches and has stuck with the last one for over a year now. Simply because, she likes him and she is getting better and better.
Alot of the pitching coaches have great knowledge and plenty of successful students. But if you go with a "great" one and your athlete is not improving, dont be afraid to try a new one. Having said that, dont be too impatient, mechanics, concepts , etc. take time.
And remember going to a great pitching coach once a week guarantees NOTHING. It's all on the athlete...desire, work ethic, mental attitude, and a little gift from god helps too.