The opening chase has a small but impressive show of magic from Sunset Shimmer — Twilight teleports in front of her to stop her as they run down the hall, only for Sunset to teleport herself, not only evading Twilight but not teleporting her cloak with her, leaving it to fly forward and into Twilight's face as she reappears behind her and keeps running. Keep in mind that the only other ponies we've ever seen magically teleporting prior to this have been Twilight herself and Celestia, implying that it takes a fairly advanced spell. Doubles as a Call Back to Twilight's confrontation with Nightmare Moon in the series premiere, demonstrating early on that Sunset is her Evil CounterpartFoil.
The final showdown between Sunset Shimmer, Twilight, and the alternate mane six. Even some of the less enthusiastic viewers thought it was pretty good.
And of course, Twilight delivering an epic Shut Up, Hannibal! to her, reminding her what most the villains of the show always seem to forget:
Twilight: Together with the the crown, they create a power beyond anything you can imagine, for it is a power you aren't able to control! The crown may be upon your head Sunset Shimmer, but you cannot wield it, because you don't possess the most powerful magic of all; the magic of friendship!
And culminates with Twilight making this simple but profound statement.
Here and in Equestria it's the only magic that can truly unite us all!
When Sunset Shimmer gives Twilight Sparkle a Sadistic Choice of going home and giving Sunset the crown or being stuck in the human world forever, Twilight chooses the human world over Equestria. While Twilight will be sad that she won't get to see her friends and everypony she cares about in Equestria, she has complete faith in them being able to protect Equestria without her or her Element of Magic. If she leaves the human world, however, Sunset Shimmer would go completely unopposed with the power of the Element of Magic just as she had without magic for years. She'd rather sacrifice her happiness with her pony friends if it means stopping Sunset from having her way.
The initial confrontation between Twilight and Sunset in the human world. She might be new to the school. She might be awkwardly trying to get used to her new human body. But no one is bullying anyone on Twilight's watch, especially Fluttershy. Not even someone with as much control over the school as Sunset.