1. Magical Mystery Cure
Yep, that's one controversial! Episode is good itself, it has best songs in the show, interesting idea (switching mane 6 cutie marks) and touching moment (in Celestia's ballad), but it wasn't character development we needed... Twilight doesn't needed wings or princess title at all (twilight will not outlive her friends btw), she could become friendship ambassador and teach friendship around Equestria and further (just like she did in all her cutie map episodes), and then become the ruler of all Equestria, because she is able to reunite everyone against main antagonists. But noooo, they gave her a wings, then they made whole season to give her a castle and etc.
2. School daze pt.1/School daze pt. 2
THIS explains whole "school of friendship" idea
I can't imagine mane 6 as school teachers. I just can't. I left a fandom after I saw leak with "school of friendship" back in 2016-2017.
Of course unite all species once and for all is a good idea, but like that? That's a nonsense. You need to teach friendship to all species to success, you should fix their internal struggles first, and ponies should meet their culture too, you're not just choosing 5 random kids, sending them to Ponyville and fixing everything by that! New 6 is a bad idea, just because there is more than 6 races in Equestria.
Antagonist is pretty weak too, Neighsay didn't even felt like antagonist, he is just reasonable, he tried to save us from this bad idea
*school of friendship is disapproved*
3. The Crystalling - Part 1/The Crystalling - Part 2
is one of the worst ways to make mlp premiere
Cadence and Shining finally have a baby, Twi is an aunt now, Starlight and Sunburst are reuniting... If it wasn't a premiere, it could be one of the cutest mlp episodes ever! But it was a premier, we needed a threat to the world, so...
Cadence and Shining Armor's child is an alicorn! (why Hasbro why why why WHY ) And she just broke crystal heart with her cry! And now crystal empire is doomed because citizens will be froze to death! Whole s6 premier issue is not an antagonist, but annoying Mary sue child, yup. 4.