Twilight realizing that Celestia used her, and coming to hate her for it, is heartwrenching. Made worse by the fact that Celestia knows this is going to happen, and is devastated by it.
Just try to get through Terra's flashback sequence in chapter 19 with dry eyes. I dare you.
Rarity's Disney Death. Her last act is to give Twilight her only possession. Vorpal.
Twilight realizing that Celestia disguised herself as Luna to get one last conversation with her estranged student. And she didn't use the opportunity to manipulate Twilight, either; she just wanted to make sure Twilight was okay. Twilight's last words to her friends before she risks her life and becomes a godare: "I love you. All of you. And I love Celestia, too. Make sure to tell her."
The Mane Six introduce themselves to La Résistance. Rarity introduces Rainbow Dash as "the fastest pony alive" and this conversation occurs.
“Fastest pegasus alive, eh?” said Noble. “How come I’ve never heard of you?” “Because I was a weather pony.” “And why were you a weather pony?” Dash looked down, and for a moment Applejack thought she wouldn't answer. “Because there’s something wrong with me,” she said quietly.