There is an art to learning to teach. I have taken lessons from some great professionals and some of them have been able to impart WHAT they do in a way I could understand it and others have only been able to show me the results without a clear understanding of how to teach me how to perform to get those results.
Jose Parica - he would show me things with the cue ball and tell me to do it and I would complain that I can see what he wants but have NO IDEA how to do it.
Rafael Martinez on the other hand was able to explain the different ways to hit the cue ball in such a way that I could grasp the mechanics and improve my touch tremendously.
Other instructors who aren't quite so high a level of player have helped me with my stroke, jumping up, staying focused, pre-shot, and all the things that a good player needs.
I myself am an instructor of sorts in the specialty of jumping balls. When I started selling Bunjee Jump cues I realized that I needed to be somewhat proficient to be able to demonstrate the cue effectively. So I started to study the mechanics of what happens during a jump shot and the mechanics of the player as he or she is executing a jump shot. As I started doing the demonstrations successfully people naturally wanted to try the great new shots they were seeing.
But, what happended? They were having difficulty performing those shots or even being able to jump the ball at all. So I had to learn to teach.
This took me a while to acquire the skills to be able to quickly evaluate and isolate the problem that an individual was having and to be able to correctly communicate the solution. Belive me, this is no easy task on both fronts. You can have two people with the exact same problem and the same answer might not work for both of them.
There have been times when I wanted to tell someone that they were hopeless.

But many times my friends came to the rescue, Jerry Olivier, Leslie Rogers, Chris McDaniel and others to impart their instruction and that was what made the difference for the person I wasn't getting through to.
Now, I am capable of teaching anyone to jump balls correctly and accurately within the span of half and hour. I regularly instruct players who are much better than myself and refine their technique.
I am surprised no one mentioned Bert Kinister. I don't know how well he plays. Also how about Tim Miller, the Monk? How good does he play? Or Bob Henning? I have seen him take down some matches against top notchers from time to time.
I think though that my vote for the strongest person who is primarily an instructor first and a player second would go to Joe Tucker. Having said that I am basing it purely on nothign more than my opinion which has no basis in facts as I don't know the history and records of all the instructors out there. I just like the fact that Joe shares so much with the world for free and he takes the time to come up with truly useful tools that work AND he can play.
(ok Joe, there's your free promo now send me my Aim Trainer)

Just kidding the preceding advertisement for Joe was completely without his knowledge.