Moondancer

I really loved "Amending Fences", both for its clever and fun use of continuity from the pilot episode, and also for being one of the few Twilight slice-of-life episodes that we've gotten since she's turned into an alicorn.  It did a very nice job of bringing Twilight and Spike back to their roots and giving them a friendship issue that was both deeply personal and still very relate-able.  

Moondancer was a big part of that...  I've seen people drawing parallels between Moondancer and Starlight Glimmer, as both characters have strong reactions to being jilted by an assumed friend in childhood, yet Moondancer's story worked much better for me.  It certainly helps that it was the less drastic and more grounded of the two... plenty of people stop putting themselves out there in an effort to avoid being hurt, while relatively few start cults and end up hellbent on time-traveling revenge, so there is that.  But I liked the twin components to Moondancer's harsh lesson at her party:  It wasn't just that Twilight went off without ever saying goodbye, it was the realization that any connection that Moondancer felt had never really existed at all.  In a different kind of story, Moondancer's belief in a non-existing relationship would paint her as a crazy person, a stalker who saw things that weren't there.  But the episode keeps a good deal of sympathy and understanding with her instead.  It doesn't seem unreasonable that someone with her limited social skills would think she shared a connection with another filly who had her same interests and drive.  Unlike Twilight, she needed a friend at that point in her life and thought she had one.  To find out that she barely even registered in Twilight's awareness was clearly devastating.  It's easy to feel sympathy for her, and it's no surprise that she's been a popular character among the fandom.  It was a treat to see her attending Twilight's lecture in the 5th season finale, and I hope that she continues to pop up as the show goes on.