Ah, Lesson Zero. A personal favourite of mine, and perhaps the episode that most reinforced Twilight as my favourite pony, even if she didn't conduct herself the best. It's an episode that continues the weirdness of S2 in the best way possible, and it's one episode that is beloved by pretty much everyone.
The episode is almost immediately put on top with a series of great jokes. Funny moments abound for much of the episode, coming largely from the timing of different characters' responses to Twilight's attempts to "help". Around midway through, though, the episode famously goes off the deep end. It was already a bit crazy, but this is a different league of crazy. Twilight's stress levels clearly come through in her actions, and said actions are... a bit unsettling. Despite having watched it several times, Twilight's "crazy mode" still disturbs me. It elevates the episode to one of the show's most memorable, and is hard to look away from.
At the very centre of the episode is Twilight Sparkle, both the protagonist and the antagonist. Her extreme stress over something silly is something that many viewers (including myself) will find relatable. She takes it to severe extremes, but her state of mind is something that may hit close to home. As her actions becomes less and less okay, she takes on a more villainous role. Indeed, late in the episode she becomes quite the threat to Ponyville. For someone that's protected the city in the past, it's quite the change of fortunes.
The result is an episode that elevates itself far above the pack. A lot of the lines are among the show's most quotable, still being remembered among fans to this day. Meanwhile, with every joke landing, the episode is the show at its funniest. Really, though, it's when the episode goes bonkers that sets it apart. Few episodes have tried something this crazy and fresh and just ran with it, and because of that it's one of the most memorable and enjoyable episodes of the show. It's telling that not one, but two episodes have attempted to recreate its success, to lesser results. In the end, there's only one Lesson Zero.