I don't think I had a "class" that really stuck with me in the ways most have mentioned. I am a history buff so I really enjoyed any history classes but I can't think of any single class that particularly sticks out. My confidence, or persona so to speak has mostly been shaped by my military experiences or schools I have attended. The most dramatic change for me took shape in the military and helped me realize I was no longer a child but an adult and that it was time to start acting that way.
I suppose I should have paid more attention in my math classes, but I suppose I was smart enough to marry a woman with a degree in Math. I hate math!
Then again, high school ROTC did start me in the process of considering the military as a career so I suppose that class was a factor in my life.