I won some swimming race as a kid in my swimming lessons, and my winning the race allowed me to move up to the next swimming lesson level.
One year where I played town league basketball (pretty sure it was 5th grade) we won the championship. That was neat.
I scored a hat trick once in roller hockey. That was pretty neat since I usually never played forward in my 8 or 9 years of playing (I started playing center towards my last few years because I was good at face-offs). It didn't hurt that the kid setting up those goals was easily the best player in the entire league. The dude had the power of Chara, the accuracy of Stamkos, and the passing of Oates for that league.
But what I'm most proud of is winning a roller hockey game as goalie in 6th grade. Our team was horrendous that year and it was the only game we won. I only let in 3 goals (which was good since a lot of the games were high scoring like 8-6 or stuff like that).
"Don't let the fat lady sing until it's absolutely necessary." -Butch Goring