There's quite a number of scenes in Friendship is Magic that deal with our ponies making huge decisions that shape the course of their futures or the destinies of others. One of the most powerful and prominent to me comes from the Season 5 finale, The Cutie Re-Mark, when Twilight Sparkle, along with Spike, manages to convince Starlight Glimmer to give friendship another chance, after showing her what happens to Equestria if she keeps trying to change history and prevent the Sonic Rainboom that connected the Mane 6 from happening. Starlight, after trying her utmost to ruin Twilight's life out of spite for making her give up her life running a village of forced equality, finally accepts Twilight's offer, a choice that led her to becoming Twilight's pupil, giving her a new lease on life as she goes on to reunite with Sunburst and make new friends. Such a powerful moment from an epic season finale, and one that paved the way for Starlight to become one of my favorite characters of FiM. =)