Mine is 'Welcome to the Show'. And it's not just cuz of the Rainbooms' Battle, but the quality of their song. It's not about how good it is, it's about how much their friendship means to them in the song.
"Under Our Spell" is great! My favorite in that movie along with "Tricks Up My Sleeve" and "Awesome as I Wanna Be". The latter is probably the best of the three for me – I always like Rainbow Dash's songs for some reason!