I feel like Rainbow Dash, Pinkie Pie, and Twilight are more well known and more vital to the series than Rarity, Applejack, and Fluttershy.
For reference, the former 3 actually have an impact on Ponyville/Equestria. Twilight is a fucking Princess, Rainbow Dash controls the weather, and Pinkie Pie preserves happiness.
The other three seem to be more about their own personal interests.