This is by far my favourite episode, there's a few reasons for this.
1. Discord.
Discord is completly hillarious, while also beeing smart and evil at the same time. He is even more random than Pinkie, and that's saying something. This is his first appearance and layed the bases for his appearence in later episodes. Discord was already at this point my favourite antagonist and just continued to get better.
2. The Mane 6 after the maze.
The reversed personalities of the Mane 6 ( sans Twi) are great. They all are very funny, and they managed to not force anything onto them. Flutterbitch in particular I find nearly as hillarious as Discord himself.
3. The pacing.
The pacing was actually ( once again in my opinion) very good. Of course part of that is that it is a two-part episode, but also the pacing in each individual episode is better than in most others.
4. The humor.
First of I really like M.A.Larson and his humor. He has a quite dry style that just always cracks me up. Especially the scene in the library after Twilight found the Elements of Harmony left me in tears of laughter for about 5 minutes.
This all adds up to a great episode.
(Keep in mind that everything I've written is in my opinion. If you don't agree that's OK, but I love this episode)