To avoid the obvious winners (This Day Aria, etc.), I really, really, really like "Oink Oink Oink". That was my favorite moment of Baby Cakes.
Community songs, I love "Summer" by Griffin Village, "Real Magic" by Tombstone, and Miu's cover of "Want it, Need It". Also everything by MandoPony. I love him.
And just because, I love "Call upon the Seaponies" and "I'll Go It Alone" from Rescue at Midnight Castle and My Little Pony: The Movie, respectively. Sorry, I grew up with ponies.