Season 9.

So let's get it outta my system - I don't like season 9. No, at the level of individual episodes it not that bad. The problem starts at the overarching plot - this whole transfer of power.
First of all there literally no explanations why the heck Luna and Celestia chooses to step down. No one even bothered to give them believable reasons to do this. It's a change for the sake of change - the whole Celestia speech at Twilight coronation pretty much this.

Second - Twilight more or less scrap everything she achieved or strived to achieve. Castle of Friendship, School of Friendship, Ponyville, friend, everything. It's simply doesn't matter anymore. She doesn't have time to teach at School of Friendship but she has time to teach at SGU? Really?

Third. Friendship. For nine seasons show tell us that M6 friendship is special, it's powerful, it's magical. But in the end? It's actually not that powerful and not that special. Instead of finding some ways to rule over Equestria and still have her friends near Twilight leave Ponyville, M6 and return to Canterlot. They meet once a month. Most likely if Twilight doesn't have others, more important things to do.

Fourth. They pretty much turn Twilight in season one Celestia only with much worse-looking magical mane. She rules alone, she controls Sun and Moon, and she has an anti-social pupil which she needs to teach about friendship... And before you know it she starts to gets her ass kicked on regular occasions.

And to add insult to injury - they pretty much scrapped Celestia (well, Luna too). For someone like me, who like Celestia the most out of all ponies... The showrunner isn't even bothered to show Sisters in the final song collage.

PS. People said that animators had plans to add Sunset in the final song collage - she becomes a Principal Celestia replacement. Feel bad for Sunset - Twilight get a more or less empire to run while Sunset ended up as a principal of some barely important public school.