For me it'd probably go something like this:
Firefly
Applejack
Glory
Gusty
Moondancer
Cotton Candy
I agree that if there had been a 'core 7' in G1 tho it probably would have been the ponies that were heavily featured in the series, something like this:
Wind Whistler
Fizzy
Gusty
Surprise
Lofty
Sweet Stuff
Shady