Rainbow Dash's interactions with her fans, especially how Spitfire describes it.
The Cutie Mark Crusaders reuniting is also quite touching (even if they've all got their Cutie Marks by now) and shows that, like the Mane Cast, they're True Companions too.
CMC: "CUTIE MARK CRUSADERS AUNTIES! YAAAAYYY!"
When the Mane Six are assembling their gifts for Windfall, Rainbow Dash is unimpressed with hers; unlike the others, who made their gifts, she brought a Wonderbolts doll of herself anyone could have bought. At that point, Pinkie Pie goes out of her way to tell RD that it took just as much effort to get that gift as it took the others to get theirs; Rainbow Dash had to become a Wonderbolt to have that doll made, after all. A little moment of Pinkie Pie being Pinkie Pie…and never letting her friends feel bad about themselves.
The last spoken line of the story "And magic makes it all complete," just about makes one's heart explode. Twice.