This is a tough one! So many great options in every series.
G1: "Call Upon the Seaponies" (Rescue from Midnight Castle)
Tales: "Off You Go" (Gribet)
G3: "Friendship and Flowers" (Princess Promenade)
G4: "You'll Play Your Part" (Twilight's Kingdom)
EQG: "Welcome to the Show" (Rainbow Rocks)
Fan-made: Between "Lullaby for a Princess" by ponyphonic and the symphonic metal opera "Moonrise" by The L-Train, it's too hard to decide--Guess it depends if I want to cry or headbang...