Season 1 has a special charm, simple perhaps to a fault
Season 2 keeps that original charm while letting more complex stories and more character development.
Season 3 was a filler season, pretty uninteresting.
Season 4 was the peak of storytelling complexity in the show, less innocent and wild. Twilight's Kingdom is the highest concept point of the show.
Season 5 was a continuation of 4, the point were the show was definitely mature, more stiff and already set in its ways but very self-confident. Imho the show could have ended with The Cutie Re-Mark on a good note even if many already disliked this season.
Season 6 was the beginning of the decline, most characters' story arcs had already concluded and there was little if any room for growth left, so the season was just a series of forgettable filler stories with little consequence.
Season 7 was imho better than 6, I remember it being funnier and having some fun episodes with the gimmick of sending two mane 6 to some place to fix a problem that highlights their differences. However even at its best it doesn't really have the magic of the earlier seasons.
Season 8 was when the true rot started and the show became borderline unrecognizable. The magic school was hideous, the new discount mane 6 were extremely boring and even the mane 6 not only stopped growing as characters but essentially regressed to just their main gimmicks, and much stiffer and less funny than they were in S1. The kirin were fun and I liked Cozy Glow but that's it.
Season 9 was bad enough that I genuinely can't recall a single episode other than the ending and the ending sucked. Gathering up the villains from the past sounds good in principle but the execution was awful because they played the villains as jokes, unlike previous villains which were played dead seriously.
The thing that bothers me the most is that ending the show by mirroring the start of the first episode should have been amazing and yet it was done so poorly I couldn't enjoy it.