For me it would have to be during a sparring match with someone much older, much more experienced and much higher rank than me. It was back when I was green belt, and this guy was like in his 30s maybe 40s with a black belt (can't recall what rank) - this what TKD by the way. But halfway through our round, I was looking him right in the eyes and I saw the most imperceptible change in his expression. We were both sort of taking a break; still in our fighting stance, circling like sharks, ready to react and attack at any moment, but catching our breaths. I'd had a lot of experience in martial arts before, and boxing sparring but this club was where I actually got to apply the kicks I learned before hand. The expression I saw was one of pure curiosity - he was trying to figure out what I'd do next, how I'd react, how I would attack. It was so imperceptible, anyone (me included) could have missed it. I think it was his eyes narrowing or something much smaller. But as someone who has, unfortunately, a big ego and have always wanted to be acknowledged for my skill, that moment meant everything to me. As soon as I saw, it was like a trigger for us to get back to sparring, like in those western movies where the tiniest sound sets off the draw.
It might sound sad, but that is easily one of my most memorable moments in martial arts, and also one of the moments that brought me the most happiness in my life.