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Chart posted for context. I will give a few options I each find valid as an important turning point, and explain which one I find to be by far the most important change. Feel free to argue for either of these 5 as the most important, or to propose your own turning point aside from these.
Canterlot Wedding: The end of the Faust era and the beginning of the McCarthy era. The introduction of Shining Armor (who was never mentioned before and was not part of the show's plan) and Cadance (Celestia & Luna were supposed to be the only Alicorns) represented a distinct shift in canon away from Faust's original vision, paving the way for Twilicorn.
Magical Mystery Cure/Princess Twilight Sparkle: This was the one shift I left out of my chart, since 5 eras was already complicated enough. But I basically see Canterlot Wedding+Season 3 as a transition between the Faust and McCarthy eras. While Faust herself was gone and the show's focus had shifted, Twilight was still an unicorn and there was a very similar comedic tone to Season 2. Magical Mystery Cure represents Twilight's status and role in the show permanently changing, while Princess Twilight Sparkle represents the show's tone shifting to be more mature.
Twilight's Kingdom: This episode represents the conclusion of Twilight's character arc which defined the first 4 seasons, as well as the Castle replacing the original tree home. Due to this Season 5 had lacked the show's purpose and direction it had before, Twilight herself become stagnant and boring (which the obvious exception of Amending Fences). There were loose ends tied up with the other characters, but they mainly felt fully developed post this episode.
Cutie Re-Mark: In my view, this episode represented by far the most significant change of the show in quality and tone. Although I very much recognize the substantive changes of the 3 episodes above, I also believe that MLP's quality stayed consistently great throughout the first 5 seasons in spite of changes in focus and tone, and it did maintain a sizeable amount of the original staff throughout those seasons. In my view this episode is the singular point where MLP changed from a great show to a mediocre one, is the point where the original staff were almost entirely gone, and Starlight's redemption was itself a major change. These factors make this the show's singular jumping the shark point in my view. Seasons 1-5 had the original writers and were great, Seasons 6-9 did not and were mediocre. It is that simple for me.
School Daze: School of Friendship is abruptly introduced, along with a new cast of side characters. Most fans I've seen acknowledge the show's quality as declining further past this point due to this forced concept along with worse episode quality, increased OOC issues, and the show utterly generally being aimless and unable to commit to developing its new concepts & settings.
Canterlot Wedding: The end of the Faust era and the beginning of the McCarthy era. The introduction of Shining Armor (who was never mentioned before and was not part of the show's plan) and Cadance (Celestia & Luna were supposed to be the only Alicorns) represented a distinct shift in canon away from Faust's original vision, paving the way for Twilicorn.
Magical Mystery Cure/Princess Twilight Sparkle: This was the one shift I left out of my chart, since 5 eras was already complicated enough. But I basically see Canterlot Wedding+Season 3 as a transition between the Faust and McCarthy eras. While Faust herself was gone and the show's focus had shifted, Twilight was still an unicorn and there was a very similar comedic tone to Season 2. Magical Mystery Cure represents Twilight's status and role in the show permanently changing, while Princess Twilight Sparkle represents the show's tone shifting to be more mature.
Twilight's Kingdom: This episode represents the conclusion of Twilight's character arc which defined the first 4 seasons, as well as the Castle replacing the original tree home. Due to this Season 5 had lacked the show's purpose and direction it had before, Twilight herself become stagnant and boring (which the obvious exception of Amending Fences). There were loose ends tied up with the other characters, but they mainly felt fully developed post this episode.
Cutie Re-Mark: In my view, this episode represented by far the most significant change of the show in quality and tone. Although I very much recognize the substantive changes of the 3 episodes above, I also believe that MLP's quality stayed consistently great throughout the first 5 seasons in spite of changes in focus and tone, and it did maintain a sizeable amount of the original staff throughout those seasons. In my view this episode is the singular point where MLP changed from a great show to a mediocre one, is the point where the original staff were almost entirely gone, and Starlight's redemption was itself a major change. These factors make this the show's singular jumping the shark point in my view. Seasons 1-5 had the original writers and were great, Seasons 6-9 did not and were mediocre. It is that simple for me.
School Daze: School of Friendship is abruptly introduced, along with a new cast of side characters. Most fans I've seen acknowledge the show's quality as declining further past this point due to this forced concept along with worse episode quality, increased OOC issues, and the show utterly generally being aimless and unable to commit to developing its new concepts & settings.
