It's a toss up for me between "Take on Me" and "Baby Shark"
Take on me was the first year I was a Nats fan and that was so much fun to sing all the way through the NLDS for all the good it did us.
Baby Shark was a whole different monster and it was the theme song of my return to the DC area and my first season as a STH..... Oh, and we sang it all the way through the World Series.
I'm gonna go with Baby Shark but Morse and Parra are still some of my favorite Nats just because of the fun they brought to the ballpark.