I liked it, but it's hyperbole to call it the best two-part opener in quality. Instead of dissecting good and bad only, I'll only list and partially dissect what I liked and not, although the two will overlap.

What I liked:

1. The humor with Twilight not being able to fly. As annoying as the retcon is, it gave the spots nice humor. And at least her difficulty was solved at the end.
   
2. Discord played a role in the story. He was his chaotic self and hilarious to boot. (The Aladdin reference with Discord taking the bath cracked me up.) He was a complete dick, and that's great to keep.
   
3. It was nice to see all the worldbuilding in the episode, from the origins of the Elements of Harmony to them finally being stored again.
   
4. Twilight saying her new role in Equestria may test her friendship with the others. That's exactly something the writers need to do in order to genuinely make her role worthwhile.
   
5. The Elements of Harmony not being used anymore. In the first two-parters, the EoH were crucial, but they began to become plot devices the Mane Six forced themselves to rely on. (This includes the lazy mess known as Equestria Girls.) With the Elements now back in their rightful place, they'll have to solve their problems with their innate abilities rather than a plot convenience. It's a completely different direction that surprised me, and honestly, they needed it desperately
   
6. The Sidekick Five actually had importance instead of being figurative background ponies.
   
7. Twilight had a genuine reason to be in Canterlot: preparing for a Summer Sun Celebration, but this time one about actually putting the turmoil of Nightmare Moon behind Celestia. (She actually got genuine characterization.)
   
8. The animation really stepped up here. It was very fluent with nicely done lighting and depth perception, further proving how Flash is a respectable medium for animation.
   

What I disliked:

1. Twilight's struggle to fly was completely unneeded (a slight criticism I had for EQG, too), and it's annoying to see her ability to fly well from the end of MMC retconned. Trade the humor from Twilight's poor flying with maybe her poor ability to land (which was done).
   
2. Wilhelm Scream…NO!
   
3. The alicorn potion is a complete Deus Ex Machina, and it became crucial in so many points. A contrived plot device and major writing shortcut.
   
4. The Tree of Harmony (referencing the Yggdrasil) just came out of nowhere. No history was truly explained. Twilight just found out about it, and that was that.
   
5. The flashbacks were pretty underwhelming and paced too quickly.
   
6. Tara Strong's voice acting upon landing before the injured Celestia felt very artificial.
   
7. McCarthy's pace for this episode was inconsistent. It went from being very slow to too quick, and the breakup was pointless and ate up too much time in the script.
   
8. Speaking of the breakup, Applejack was very out of character at that moment. She wouldn't be the one who'd suggest Twilight to return to Ponyville. Maybe someone like Rarity or Fluttershy, but it doesn't feel right coming out of the cowmare's mouth.
   
9. The plot tells far too much, telling the audience how the writer and rest of team had trouble fitting it all in forty minutes without cramming and mishandling the pacing. Show, don't tell.
   
10. The idea of the Elements of Harmony being predestined (Twilight's cutie mark on the Yggdrasil and Element itself) is quite bothersome. Destiny doesn't choose you. You choose your destiny. It was a major criticism I had with Magical Mystery Cure, and it's the same here.
    

In all, it's a good episode. It's better than The Crystal Empire, but worse than Return of Harmony (best two-part opener) and the pilot. PTS needs a lot more editing (fix the pacing, show more rather than rely on an obvious DEM and exposition) to solidify it and make the writing quality convincing, and the two-parter doesn't genuinely prove the Twilicorn's worth as princess at this point. Twilight has a ways to go, and DHX needs to show in the canon she earned it. It's on the right track, but Twilight's ascension isn't currently convincing. Hopefully, future episodes change that.