I’d say my first full-time martial arts teacher, because I was young enough to have considerable open domain in which said teacher could be influential and because the lessons were so diverse — from kinesthetics to ethics to culture to psychology.
By “most influential” I think one means having had either the broadest or most profound influence. (I favor the importance of the latter, the lessons that stick with one and which inform one’s philosophy of life.) This definition favors earlier teachers, but as I don’t remember any specific lesson taught by a specific elementary school teacher, they’re out (though I learned useful things from them and they ranged from competent to quite skilled as teachers go.) The earliest profound lesson I received through scholastic education, one that became a core tenet in my philosophy of life rather than just a skill, was in high school — and in my junior or senior year at that.
As with books, if I can take away one profound lesson from a teacher, I consider my experience well worth the time and energy. And most influential isn’t necessarily the prime criterion for a teacher — and certainly doesn’t necessarily mean best or most skilled or most in command of a diverse array of knowledge.
Tiger Riding for Beginners 