Forgotten Friendship is the only thing in the EqG library that threatens Rainbow Rock's throne for me, I could see my self going either way on certain days.
Forgotten Friendship is, to me, Sunset's "Amending Fences", it's her Hero's Journey completed. Not only in the sense of her coming home to make amends with Celestia (which is a beautiful moment) but also in just showing how far she's come since the beginning of the spinoff. She's put into the same position as she was in the first movie, sans friends and on her own, but with all the lessons she's learned since then and it's a gorgeous pay-off to see her reclaim the relationships that have grown to mean so much to her. Her dynamic with Trixie is golden and Wallflower is really good antagonist for this scenario, it's peak.
It's also really interesting in how, even with how this story can be seen as tying a perfect bow to Sunset's arc, it also doubles as the beginning of an entire new saga that would be the latter half of EqG: it introduces the cast, Sunset's past, puts her strengths and flaws on full display, and sets up new viewers for watching the continued adventures of horse-girl high school.