Like others, it's difficult to name just one. I was fortunate to have had really good teachers throughout my academic career. As a group, I'm particularly thankful for all the English teachers I had at my local school system. Thanks to them I was able to test out of all my college English.
My high school chemistry teacher was the most difficult, demanding teacher I had until college. His class was hard, but he was a good guy and very fair. He prepared me more for college than any one teacher.
In college, I had two professors who were responsible for me graduating. My differential equations instructor was the single best teacher I ever had. I was fortunate to have, by luck of the draw, a phenomenal profeesor who taught the most critical math class in all of engineering.
The last was a good professor, but an even better person. I had four different classes under him, including our make-or-break "senior design project." For some reason, this gentleman took a liking to me early on, and somehow just happened to be around when I was struggling, down in the dumps, or just plain frustrated. "C'mon Duke, let's go down to my office," he'd say. This happened maybe four times, and he somehow seemed to be able to send me on my way with a different perspective on what ever had been bothering me.