By the measure of having taught still useful lessons about HOW to think (those who taught me WHAT to think are largely forgotten along with their lessons,) that would be a tie between my 11th grade Psychology teacher and an undergrad Religious Studies professor. The former, among other ideas, first exposed me to what I would come to believe is the most important lesson of human existence under his label of the “gestalt of expectations.” [I’ve never heard anyone else refer to it as such, but the lesson was sound and I would latter find it in philosophies from Buddhism to Stoicism.] The latter teacher, among other ideas, exposed me to two common opposing modes of fallacious thinking, what he called “the outhouse fallacy” and “the first-est is best-est fallacy” (he was a folksy, if erudite, professor.)
In terms of personal growth and development, generally speaking, there are numerous teachers of martial arts, yoga, and other mind-body practices that are incomparable and thus unrankable. Not to mention, a sound argument can made for the repugnantly unhumble statement that I am my most influential (and most important) teacher. (I state this claim not as though I am unique, but as one that could apply to anyone.)
Tiger Riding for Beginners 