As a player
Just got called up to High-A and was playing against an old teammate of mine. First professional game my mother, who taught me baseball, got to see. I hit a bomb over my old teammates head in left, and when I crossed home, my mother was standing right next to our first base dugout, tears in her eyes...at least I assume so, she had the camera goin.
As a coach
This isn't a happy memory, but it's one of the most important. When I was 19 I traveled around England teaching kids at schools how to play baseball. At this one school near Plymouth there was an 11 year old girl who was just a natural. These kids had never even heard of baseball, lt alone play, but this girl looked like she had been playing since birth. Even the rules seemed natural to her. My partner (33years old at the time, used to play in the Yanks organization) and I really encouraged her to play in a local league. She really took a liking to my partner, we figured she had a little crush. At the end of the day she came up to him with tears in her eyes and asked, "Can you come home with me and be my Dad? My Dad doesn't talk to me. You can teach me baseball everyday."
My partner was in tears driving back to London.