The "best" hitting instructor is a difficult question to answer. I think it would depend greatly upon the age, existing skill level, and committment of both the player and the parents (who pay the $$$).
I have found the folks at Frozen Ropes in Glen Burnie know their stuff pretty well and have a great facility.
No question that "best" is a relative term though and that the best instructor for one player may not be the best for another.