Worst episode: Daring Don't. Terrible showing for Rainbow Dash, and Daring's actions made little to no sense. The second half was slightly better, but not enough to compensate. It's not alone on my lowest tier list, but it certainly is the one that first came to mind. Dishonorable mentions: Trade Ya!, Princess Spike, Somepony to Watch over Me. *Cough*Amending Fences*Cough*
Best episode: A two-parter takes this spot for me... Return of Harmony. Introduced a really interesting villain in Discord, a threat dire enough to make Princess Celestia the Unflappable take him entirely seriously, Fluttershy trolling the Prince of Trolls just by being nice, candy cotton clouds that produce chocolate rain, you guys, CHOCOLATE RAIN!... And of course, Tom the Diamond... this episode had it all. Honorable mentions: Every Rarity-focused episode. All of them. Fite me, m8.