I think the compliment that sticks with me the most is when a teacher in high school told me my potential was much higher than I ever let myself reach and that I would be capable of great things if I just tried. I had massive confidence issues in high school and I slacked off in pretty much every class, so when he said that I didn't really believe it. But it's something that has always stuck in my mind, and is one of the driving forces that led me to finishing university at a time when I was really close to dropping out because I felt like I wasn't supposed to be there.
The other ones are two instances in particular that I can think of, both relating to friends who I thought I didn't mean much to but they both gave me really nice compliments telling me how much our friendship means. The instances are isolated, but they're similar moments, neither of which seemed to have been triggered, so they have really stuck with me.