In a lot of ways, “Return of Harmony” works better as a finale for Season 1 than a premier for Season 2. It’s the culmination of all the friendship lessons, using them to save the day, and it balances the Epic Two-Part Battle Against Ancient Evil from the beginning of the season. Meanwhile, “Lesson Zero” actually introduces the new premise of the show.
But I actually think it’s better placed where it is, both because it leads well into “Lesson Zero” in ways we’ll discuss in the next couple of weeks, and because it is a strong enough episode to be excellent wherever it goes. “Best Night Ever,” on the other hand, would be badly overshadowed stuck between “Party of One” and “Return of Harmony.” Also, “Best Night Ever” is a celebration and reminiscence of Season 1, while “Return of Harmony” is about closing off Season 1 so that Season 2 can begin.