A few notes- An argument could be made that Firefly should be the lead shown for the first G1 series, but she only made one appearance: the 1984 television special 'Rescue at Midnight Castle,' compared to Wind Whistler who featured in the 1986 movie as well as the subsequent animated TV show. There is really no clear main pony character or 'first among equals' in the original series unlike in the successive shows. Wind Whistler is included here for the depiction of her as a leader figure in the show as well as being one of the ponies with the most time on-screen.
Also, despite not having a formal TV series, G3 had enough video specials that I believe justified its inclusion, along with Pinkie Pie as the unofficial head of the G3 Core 7.
G3.5 and G4.5 'Pony Life' are not included since they are essentially just artistic redesigns of the G3 and G4 leads respectively, as opposed to completely new characters.