Best instructor is going to be subjective. Different instructors relate differently to different kids. One persons best instructor might have screwed up another kid.
But, for my son, it's pretty simple. The best pitching instructor has been Ray Burris. Ray has a long history in the majors, but he also relates to the kids. He also doesn't coddle them, he makes them work.
Best hitting instructor for my son has been Shane Davenport at Ray Burris. Shane can throw two pitches to my son and knows exactly what his problems are.
Both of these guys also teach the kids to learn how to diagnose the problems themselves. To learn to coach yourself, because baseball is all about adjustment.
Lots of instructors and baseball academies have come and gone thru the years, RBASI was the first around, and continues to be the best in my book.
They are also the friendliest bunch of people I know. I have seen Ray work with kids that couldn't afford lessons, making either them or their parents work them off some how. I've seen Ray make trips to our local Elementary School to read to the kids as a mentor. Just all around amazing people.
http://www.rbasi.com