Best:
"A Hearth's Warming Tail" - While standard fare for a kids' series to do a holiday episode culling the plot from one of the classics, especially with A Christmas Carol being the most common (even It's a Wonderful Life and The Grinch are arguably derivative of this story), this one stands out just for MLP being MLP. The technical presentations were top-notch, with the crew doing a different location and visual style. The characters were well-cast in their classic roles, each bringing their own flavor, and the tie-in with Starlight's character arc worked perfectly.
"The Saddle Row Review" - It's nice to see Rarity's character arc continuing after achieving her initial goal - it shows how life doesn't end when you get everything you always wanted; it's always a never-ending pursuit of more and that's not necessarily a bad thing when you're putting all your effort into it. Interspersing the main plot of the episode with the interview flashbacks is a nice shakeup of the typical MLP episode formula, allowing the characters to provide commentary on events with a nice self-referential flair. I find this episode deserving of its popularity.
"Stranger Than Fan Fiction" - Continuing on the theme of self-reference, few kids' cartoons are better equipped to comment on geek/nerd culture than this one. To that end, said subject is still a relatively new concept and very few kids' cartoons in general have touched on it tastefully. Thankfully, MLP does it, tying it in with a good moral that may well have been a little bit of what the fandom needed to hear at this time.
"To Where and Back Again" - Although Starlight Glimmer's character progression didn't really get enough focus IMO, this episode makes up for a lot of it by showing her apprehension towards her past in a tense situation, as well as touching on making amends with her town; a thread that would have made for a great episode of its own but was virtually glanced over in the previous season finale and didn't even get a regular 22-minute episode this season (seriously, "Applejack's 'Day' Off" gets made over this?). Add fan favorites Discord and Trixie, Season 6 breakout Thorax, and returning main villain Chrysalis and you have a band of unlikely heroes that make for a different kind of two-parter.
Honorable mentions: "The Crystalling", "Gauntlet of Fire", "No Second Prances"
Worst:
"Flutter Brutter" - This is one of those episodes you either really like or really hate, because it came from a good place. The plot is something many of us can identify with, and such is the flaw the focus character of the episode allegedly has. However, fear of failure is the least of Zephyr Breeze's problems; his straight-up laziness and self-centered attitude are the bigger problems and have little, if anything, to do with a fear of failure. The episode proceeds to fall into almost the same trap as "One Bad Apple", but thankfully doesn't cast the Mane 6 in a negative light as that episode did with the Crusaders. It virtually absolves Zephyr of his manipulative diva attitude by blaming it on his fear of failure as "One Bad Apple" did by shifting responsibility for her bullying ways onto her lack of social prowess (never minding that she never attempted to make a real friend as much as she tried to compensate by hanging out with the cool kids). The end of the episode nearly salvages it, but it's not enough to distract me from the notion that it grossly misrepresents the state of mind that the plot of the episode is supposed to be about. Zephyr is not a character who fears failure. He is a character who dislikes responsibility. There's a difference.
"Newbie Dash" - My reasons for disliking this episode have changed over time; it's not so much the nicknames or the moral that are the issues with this episode as much as the terrible comedy and slow plot progression. If you don't have much to deal with, you at least have to make up for it in entertainment value (that's what many of the better episodes this season did), but that's not even a problem with this episode; it was paying off one of the biggest character arcs in the series, which was open since the first episode and had its payoff deferred for about three seasons.
"Applejack's 'Day' Off" - This episode really, really tried to go too far with a paper-thin plot, and I don't think I've ever seen Applejack flip from showing thrift and ingenuity to inefficiency and irrationality (within her own area of expertise, no less) in the space of a single episode. It's the kind of episode that just feels like it was burned off so they can get to 26 on the season, and with so many better stories that should be told, this one should have hit the cutting room floor.