Okay. I know you said "all time," but I've got to give two answers, because I'm not sure if "college professor" is included in "teacher."
K-12
My 9th grade science teacher, who we all just called "Doc." I was this tiny, 4'8" (at that time) freshman, and was a bit overwhelmed. As you can imagine, passing period for someone who is 4'8" in a high school of around 2000 is... not fun. looking back, I can honestly say that passing period was nearly panic producing. Like... to the point that when I've gone up to the school for events and such as and adult, I STILL cringe when I hear the bell. So after a mildly frighting first half of the day, I pretty much instantly felt at home and at peace in Doc's room. He was just one of those teacher that exuded love to his students. He was the sponsor of FCA (Fellowship of Christian Athletes), and when I inquired, but said I probably wouldn't go because I wasn't an athlete, he said, "Here, we like to call it Fellowship of Christian Anybodies!" He also set me on course to have perfect attendance all four years of high school. I still keep in touch with him, and he's still just amazing. A lot of that (the passing periods, feeling at peace in his room, etc.) I really didn't even notice until I reflected back years later.
College
Dr. Hamblin. I had had him my Freshman year for one class, and really liked him. I was an education major, and, due to field experience and student teaching, it was recommended that all education majors take their "Senior Seminar" course their junior year. I found once called "Sports and Literature" that was taught by Dr. Hamblin. I like (watching) sports (heh. See above story about FCA....), I like literature, I knew I liked Dr. Hambliln, and it fit my schedule. SCORE! (Pun totally intentional.) This class was team taught by Dr. Hamblin and another professor. ONe of the first days, I stayed behind to tell them that I technically don't meet the prerequesites for the class, since I was taking it as a Junior. He said, "Allison, I know you. You'll be just fine." Then he turned to the other professor and said, "Have you ever met Allison? I've had her in class before and she's a good writer." And I just stood there in utter shock. A college English professor just called ME a good writer. Shortly before this, I had written something that I thought I liked, and this comment pushed me to join a Christian writing community that has meant a lot to me ever since. I have even had some of my stories published in books produce by this writing community, and have had the opportunity to give Dr. Hamblin a couple of the books and keep in contact with him too.