I have to say one of the most knowledgable hitting instructors that I have had the pleasure to listen to is Justin Stone (White Sox Academy- Lisle, IL). I had the pleasure of listening to him present this past weekend at the Wheaton North High School Baseball Coaches Clinic and he did a great job, like always.
He breaks down the swing better than anyone I have ever heard speak (7 years of attending clinics) on the topic in person. After he concluded speaking, I leaned over to a fellow coach, who has professional baseball experience and over 10 years of coaching experience, and he agreed that Justin was one of the best he has encountered.
I am sure that his knowledge and communication skills make him an excellent instructor.