The reasons are many. To sum up most of them, it's a fun episode with a well-written plot that is logical and balanced, comical, and tells your staple MLP story about character growth, etc. But I confess that the prevailing reason why the episode has so long remained the unchallenged most favorite is the more bias one.
I'm a sucker for time travel stories.
Why do you think I'm a Whovian, among other things?
It helped that the very idea of MLP telling a time travel story struck me as such an unlikely one that, when it did happen, and was so wonderfully rendered too, I was rather blown away that it had taken place. It had struck me as the one episode where MLP opened a can of worms it normally didn't, and the implications and full ramifications of that were...intriguing. As such, the episode has always had a special interest with me, and still does.
But the one thing I thought the show needed to do, it was come back to it; "It's About Time" opened that can of worms that is time travel, yes, but it was clear it would take a whole new episode to explore where that can of worms could go next.
So in a way it kind of figures that the episode that can actually rival "It's About Time" as the top favorite is the one that does precisely that.
It's the what-ifs.
I love a story that posits the question of "what if this had happened instead?" and then explored how very different things could be because of it. "Alternate histories" is one term for it. However this goes further than a retelling of actual history, because this certainly applies to the histories of fictional stories as well, and it's canonical storyarcs. It's fun seeing how different your favorite story could've been had that one key event gone totally differently.
And MLP is no exception. I'm sure you've all seen the comic or fanfic that explores the idea of what might happen if one episode of the show had ended differently before, and this is basically what this episode is, really. I almost wonder if the show's crew means this as something of a salute to us bronies who have already gone and explored those "what-ifs."
Whatever the case, this episode's very existence says loads about how far MLP has come, as it reveals that the show has been around long enough and developed enough of it's universe's history that it can actually afford to go around and positing these "what-ifs"; that it has enough of a timeline established that should some variable in it suddenly change, there's enough of the "hereafter" to show in full the consequences of that change.
And, oh, the implications of those consequences. Things do not end well for Equestria when you go meddling with those key events, as Twilight and Starlight Glimmer quickly find out. As such, this is easily one of the darkest episodes in the whole show's run up to now, as it's the first time the show has really touched on the less utopic elements of reality; on what would happen if some of the members of our gallery of villains actually had won.
I mean, it shows a flat-out war taking place...twice.
And it heavily implies a war having taken or taking place...twice more (trice possibly, pending on how much force Discord had to show to win in his timeline).
And I don't know what happened to cause the wasteland version of Equestria, but it certainly wasn't anything good.
Maybe that's the one where Trixie won...maybe let her ego do something that resulted in a nuclear wasteland sort-of outcome?
Although if I'm honest, that wasteland really reminded me more of Ceti Alpha V as it appeared in Star Trek II: The Wrath of K--
Oh. Snap. Shouldn't have had that thought, I do not need more ideas on how to cross Star Trek over with MLP.
"Twilight...are you sure these are the right coordinates?"
"Starlight...this is the garden spot of Ceti Alpha VI."
"I can barely see it."
...okay, okay, couldn't resist.
Anyway, it was pretty cool (well, relatively) to see what becomes of Equestria should all of these villains had won their battles, and how it would effect everypony else, in no special order too, I note. It was also great fun to see all the little things associated with these villains appear again too, all the little details the crew surely had to flip back to past episodes to get straight (such wonderful attention to detail, in fact) especially the musical motifs and themes associated with them, and I suspect the show's crew had plenty of fun making it all happen to. So much so, I was a little surprised Discord didn't get more screen time for that very reason. But then again, I suppose we've already seen much of whatever Discord would do if he emerged as the victorious villain several times before.
Besides, if it meant Queen Chrysalis actually got some screen time again, I'm all for it! Oh it was great to see her back in action again, and awesome idea to make her reveal being disguised as one of the mane six, to boot! She's a proper villain after all.
Of course, Sombra proves himself a capable villain too in this episode as well, and Nightmare Moon proves she's got a dastardly wicked streak that was pretty cool to see too.
And again, there's the attention to detail. For instance, I noticed that in a number of these "bad" futures, the Ponyville area seems overrun with forest life...suspiciously similar to that found in the Everfree Forest. Perhaps implying the expansion of the Everfree Forest from the season 4 premiere was allowed to run rampant, or was at least irreversible, in some of these bad futures?
I also noticed that when Nightmare Moon spoke of banishing Celestia to the moon (not the sun...huh), she explicitly states that Celestia had been there for "years" now.
Thus proving that years have passed since the series premiere with "Friendship is Magic" on to this episode, years implying more than two or three. So maybe a broadcast season of MLP does equate to roughly a year of Equestrian time after all?
Yikes...that would mean the CMC were without their cutie marks for upwards to five years...
But of course there's more to the episode than the time travel and the alternate timelines that compel me so, because there's also Starlight Glimmer's little revenge seeking that starts it all, and Starlight really proves herself as a villain here too, much more than most others, in fact. MLP is no stranger to the revenge stories, but Starlight's seriously takes the cake, not just because her's is the most inventive, but her's is the most emotive. Starlight is extremely passionate about how much she wants her revenge and how far she'll go to get it, and boy howdy does she ever sell it. More than once in the episode her taunting was infuriating me so much I wanted to reach in and strangle her...which is good because that's exactly what you want your audience to feel when watching these villains act. And it helped you could see the same in the expressions of other characters, Twilight especially, who I think showed a lot of self-control in not recklessly lashing back at Starlight more of the time. Proves Twilight has the self-control that somepony pushing her buttons isn't going to be enough to cause her to lose control...but then she's had plenty of practice with Discord, who seems to love to push specifically Twilight's buttons as often as he can, especially as recent episodes proved.
But it was in this episode where I really got sold on the idea of Starlight being a villain, because rile her up enough and she delivers. She is easily the best villain MLP has introduced to date, one I would love to see recurring encounters as such a villain again in the future.
...and then they reform her.
I mean seriously, I'm not against the reformation, but at least let the villains get a few episodes (more than two at least) to really get to go about and shine as the villains they are! Starlight would've been an excellent villain to do this with, as she's sort of Twilight's "evil twin" in many ways, everything I think Sunshine Shimmer was supposed to be (but didn't succeed to achieve), and she seemed like the excellent sort of villain who can pop up, cause a ruckus, and then craftily slip away again so to come back again at a later date and do it all over again.
But then again, the plot was set up in such a way that the only way out really was to reform Starlight. I mean, Twilight said it herself, Starlight had her either equally or completely outmatched in every other way, so Twilight really couldn't win the fight by sheer brute force or even wit or strategy (yet more proof of how much Starlight could've rocked as a long-term villain, but I digress). The only way out was to convince Starlight to end it, and to do that you had to reform her, or she would've had no real motive to end it herself, at least none that would really be that convincing.
So as much as I'd like to, I really can't argue that the reformation was still the right thing to do. I mean really, I can't see any other way out of that mess than to reform Starlight, though I'd be happy to consider any alternatives others might come up with.
And the song...I confess it didn't leave any lasting impression on me, but regardless, it appeared at exactly the right spot in the episode. Didn't feel forced or trampling on the storyline, hindering the overall tale the episode is telling. It is exactly how I want MLP to use the songs. To put them when they feel natural, where they don't hinder the story, and aren't overbearing or too numerous. I want every episode of MLP to use it's songs in exactly similar manner, not necessarily at the end, mind you, but where the episode is best suited for it, and still be able to carry on the plot the episode is trying to tell at the same time. If MLP did this more often, I would have far less to complain about the songs.
But yeah, this was a wickedly great episode, and one that has an almost...ending...feel to it. I mean, as I was watching that ending, I literally had to stop and remind myself that MLP was not ending with this episode, and that a season six has already been confirmed to be oncoming. Which is good, because I certainly do not want MLP to end just yet, nor do I feel it has to just yet either. I think it's got a couple more seasons in it still...perhaps more. Only time will tell, but always, I remain optimistic about the future of seasons upcoming we now have to look forward to.
But that said...if MLP did have to end here...I confess that this would've been a great way to have ended it all with.
And I think that sense of satisfaction alone says oodles about this episode.
Few final things for your consideration...does this mean Starlight is effectively Twilight's student now, like how Twilight had been for Celestia back in the day? If so...does that mean Starlight's possibly alicorn material too? I'm not sure how I feel about that...
Also. Zecora. Knows time magic. And is startlingly fluent in it, and it's complications. She speaks of it with a air of...experience.
...is there something you aren't telling us, Zecora?...