I could name several, but two stand out. The first is my high school, college prep chemistry teacher. He was the most difficult teacher I had had up to that point, he both expected and demanded hard work and excellence in his class. He taught us ways of reading/setting up/working problems that I use to this day, and certainly would not have passed engineering school without his approach. Great guy. Unfortunately he has a somewhat unique name and trying to reconnect with him has proven unsuccessful.
The second was an aero engineering classmate. We were really tight during school, but he joined the USAF and became a pilot and we lost track after a few years. A couple years ago I decided to try to find him, and discovered he was a pilot for American Airlines. When I tried to contact him early this year, I found out he had passed away. Sadly I waited too long.