My OW class was co-taught by two fabulous instructors, Ken Nemeth and Kendra Kirz (who taught Ken to dive). Ken is still an active instructor in the Atlanta area. Unfortunately, Kendra has retired from teaching. They presented the material in a manner that made learning to dive easy even for someone like me who always had difficulty with anything in the water. They gave me confidence in my ability. I didn't have ability in the water when I came to them, so I had nothing in which to have confidence. They built that ability in me from almost nothing and along the way showed me how far I'd come. They should be every instructor's role models and their methods should become common place.
The next great instructor I had was a man who never issued me a card. I never took a formal class from John Riddick, but I worked with him on dive charters for almost seven years. John is tough, he is rough around the edges, but he knows more about the practical application of diving than anyone I've ever met. He showed me why things I'd learned in various classes and in dive textbooks are not always best. My wish is that if I'm ever in trouble in the water, John will be my rescuer. The man is without peer.
Finally, I took Nitrox from a man with whom I sometimes team teach. Frankly, I taught myself nitrox long before I broke down and took the course. Mike Brady (MB on this board) was aware I already understood nitrox. He adapted his course to not only test my knowledge and to push me, but also to make sure I would have no trouble transitioning into teaching Nitrox. When I took my Nitrox Instructor course from Joel Silverstein (who wrote the nitrox text for NAUI, YMCA and the nitrox section of the NOAA Diving Manual), all I really needed was the transition to Y standards. My feeling for Joel is that he could also easily make this list, but I simply did not spend enough time with him for him to have such an impact on me.