If I were to tell you what my favorite episode of FIM was a couple of days ago, I'd say Magic Duel. Today, not anymore. Testing Testing 1, 2, 3 is now.
That wasn't the only reason (although it's easily one of the funniest in the series). Everyone was in character, and the moral was executed so brilliantly, it'll hit home to anyone. It's a mature episode done RIGHT, a track record FIM has missed quite a bit during the series.