I mean personally I like them all. 2 or 7 is probably my favorite, can't decide which. I think the charm from season 1 stems largely from this feeling of "I went into this show expecting to hate it, so why am I enjoying it", and seeing all the new fanart/jokes/etc (which was created at a speed that still boggles my mind). Also Pinkie Pie is weaponized waifu material in season 1, but becomes complete shit after season 2.
Also as a side note, it's funny that MLP, despite its premise, has virtually no "stronk gurl power" nonsense. The main takeaway is "There's more than one way to be feminine, each of which are equally valuable", which I think is a far better message to aim at girls.