Now, on to the second season!
With the second season, people expected great things. There was plenty to hope for, especially after the great run that capped off the first season. However, I can't imagine anyone expected this. Simply put, The Return of Harmony is brilliant.
Part of this does come down to the villain, Discord. His complete disregard for order allows for great humour, something that has given him enduring popularity. Of the villains, I maintain that he's probably the most entertaining of them. This is enhanced with a slight level of threat. In the episode, he doesn't show any limits to his power, nor any limits to what levels he might sink to. Sure, later we see he has standards, but here we don't know them. Discord, in this episode, is strictly antagonistic, and clearly doing things for his own amusement. This all makes for a much more interesting antagonist than S1's Nightmare Moon, who was only interesting for her backstory.
Meanwhile, we have the mane six, who one by one are defeated by Discord, with their minds warped into their opposite. This is a fairly dark idea, especially for this show, but it's a risk that pays off. These warped versions of the ponies we know prove to be quite funny, interacting with each other in suitably silly ways. Fluttershy, in particular, suddenly has a high level of snark, throwing rude comments left and right, with hilarious results.
The episode has perhaps the largest scale of any episode up to this point, with the heightened stakes of the show's pilot combined with a darker tone. This is particularly prevalent in the bombastic ending ceremony. Meanwhile, the plot is continuously gripping. Once the first half ended, I wanted to move on immediately to the second. The story is near-perfect right up to the final twist, which seems a bit like a deus ex machina but can be explained away. Of course, finally it ends with the rediscovery of the main tenant of the show: that friendship is magic. In these bigger episodes, that's always welcome.
So The Return of Harmony is probably the best opening the second season could have. With its great humour, gripping plot, risky darker elements, and larger scale, it's easily one of the show's best episodes.