I'm going to mark my progress as I watch all the pony shows. I've seen some of all of them, but I'll need to get some episode guides to know how far along I am.
Lucky for me, such things exist. I really need to keep this updated

G1
Specials seen:
Rescue At Midnight Castle
Escape From Catrina

Movie:
not yet

My Little Pony & Friends I eps seen:
10/48 eps

My Little Pony & Friends II eps seen:
2/13 eps

My Little Pony Tales:
14/26 eps
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G3
8/23

G3.5 (hard to figure since G3.5 wasn't technically a show, I've seen some of the animations though)

Newborn Cuties ("Once Upon a My Little Pony Time"):
All released eps seen (2/2)
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G4 Friendship is Magic:
65/91 eps

G4.5 Equestria Girls:
0/1