THIS IS A REALLY HARD QUESTION…so I’m going to pseudo cheat and say my favorite EPISODE involving Luna is Luna Eclipsed. For one, watching Luna awkwardly mingle with a modern Equestria is a gift to this world tbh, and I think the episode did a great job balancing the hilarious aspect of her dealing with all that changed over a 1000 years, with the more serious and sad aspect of her having to deal with the remaining fear the ponies felt towards her after Nightmare Moon.
In fact, I think Luna Eclipsed is single handily the best episode for any Luna RPer to look at. For me it’s the episode that gives you the biggest view into Luna’s personality, and what I love about it, maybe surprisingly, is that it highlights a lot of Luna’s NEGATIVE traits. She’s hot headed, a stickler for decorum and a bit more “high and mighty” than Celestia, and she’s got a pretty damn big pride. As a writer I really appreciate that, especially since most other episodes involving Luna either have her revert to a bit of a flat character (arguably the CMC dream episodes) or emphasize her positive traits mostly. I may even go so far as to say that Luna Eclipsed was the whole reason I even came to love Luna as a character so much, because unlike Princess “no character development” Celestia, the episode really makes Moonbutt a well rounded character with flaws and development. And considering Princess Luna is one of the most beloved ponies by the fandom, I can’t be alone in thinking so.