Another suggestion of mine. This is one where all 6 of them get a chance of singing the melody, so the parts aren't too specific. Though Rarity. Here are everyone's parts:
Applejack - Alto 2 (Lowest part)
Twilight Sparkle - Alto 1 (Second Lowest Part)
Rainbow Dash - Soprano 2 B (Third Lowest Part)
Pinkie Pie - Soprano 2 A (Third Highest Part)
Rarity - Soprano 1 B (Second Highest Part)
And Fluttershy - Soprano 1 A (Highest Part)