It all started with the pilot, the initial two-parter "Friendship is Magic". Contextually, this episode is important for establishing the characters, but it's important to note that this is the point where they first meet. In the earlier episodes of S1, the Mane Six have formed connections by way of the Nightmare Moon incident, but are very new friends. They have few bonds and less cherished memories, and looking back it shows in their interactions. There's a certain lack of closeness in dialogue between members of the mane six in the first few episodes, which is more similar to how minor characters are interacted with later in the series. Interestingly, this serves as a launching off point for much of the season. If you look at most of S1's plots, they're built around events that would inspire bonding between characters. "Griffon the Brush Off" has Rainbow Dash bonding with Pinkie Pie, while choosing to stick with her new friends over her old ones, "Look Before You Sleep" puts Applejack and Rarity in a situation to better understand each other, "Dragonshy" gives the ponies a small adventure through which they gain more exposure to each other's personalities, etc. By the season finale, the mane six have grown thick as thieves through their shared experiences through the season.
Meanwhile, new traits to the characters are introduced throughout the season, with the mane six slowly becoming more and more fleshed out. "Friendship is Magic" gives us merely the most basic of introductions, and it's the rest of the season that develops the characters the rest of the way. In this season Twilight has the most development, but the further in we get the more other characters are explored, until episodes start to shoehorn her in as an afterthought. By the end, we're familiar with each of the six's dreams, problems, and personalities such that when they're put to the test in "The Best Night Ever", viewers are invested. The finale to S1 is one of my favourite episodes partially because of how it serves as a culmination of everything up to that point. We've seen how much Rainbow Dash loves the Wonderbolts, so we're excited that she has the opportunity to meet them again. We know that Twilight hasn't had much time with Celestia, so we care about their interaction. And so when things start to go badly, it's compelling-though that is helped by the damn good writing of the episode. In the end, though, when everything's gone badly, the episode turns into an opportunity to reflect on how these six ponies came to be the best of friends. The consistency of that theme is something of a sort that has never really been replicated in subsequent seasons.
Still, as good as S1 is, looking back on it, it's more quaint than subsequent seasons. This is a selling point for some, but I think that makes its episodes a little less interesting individually. It's probably the least extravagant of the seasons, and it's definitely the most constrained. There aren't many opportunities to pull anything truly ambitious, though an attempt was made with "Party of One". Enter season 2: More fragmented, but far, far more ambitious. With S1's restrictions now off the table, the writers are free to explore a bunch of ideas that would have never happened in the first one. Nowhere is this better represented than the season premier, the two-parter "The Return of Harmony". This is an episode that is entirely different from what came before: Darker, bigger, and crazier. This is an episode where the main characters come perilously close to being defeated, saved only by chance. It's then followed by the madness of "Lesson Zero", the most commonly recommended jumping on point. But S2 is a veritable treasure trove of writers trying out different ideas, ranging from something as simple as Luna's return in "Luna Eclipsed" to attempting to stage an entire episode in the past in both "Family Appreciation Day" and "Hearth's Warming Eve".
Each of the episodes in S2 has a slightly different personality from the rest, and this results in them standing out from each other. This makes S2 easy to critique on an episode-by-episode basis, while also making it more difficult to critique as a whole due to its sheer diversity. However, this backfires in some cases, where the unique ideas fall flat. Let's be clear here: Where S2 sucks, it is infuriating. While S1's low points were merely dull or annoying, S2 goes the extra mile, inducing rage in its lowest points. Meanwhile, at this point the characters have moved further along in development, and they're often explored in more depth. Applejack has her flaws examined in more detail, Rainbow matures considerably, Fluttershy makes strides towards becoming more assertive, etc. There's no "The Best Night Ever" to put a lid on it, with the season instead ending with another two-parter. "A Canterlot Wedding" is an episode of such lofty ambition that it seems it was combed over to subtly close up some of its problems and there's still elements that are questionable. This is an episode that feels like a Big Deal, paced near-perfectly, flawed yet brilliant. In a way, this episode does serve to exemplify all that S2 is: in some places flawed, but diverse, unique, fresh, and overflowing with ideas.
The third season is the odd duck of the four. It's the popular least favourite, having been cut in half and subject to controversy. This is a season that attempted to reintroduce a running story, but this idea is rapidly discarded after the first episode only to reappear in the much-maligned finale. S3 has gotten flack for a number of reasons, but the biggest one is characterization. S3 has a lot of exaggeration in the characters. It's not really a consistent thing, but at points the writing exaggerates certain traits of the characters. At times, it even invents traits in order for a plot to work. In episodes like "Spike At Your Service", characters grow out of traits they never had, leaving the episode without any actual net value. Meanwhile, a lot of the episode's experiments don't pay off. This isn't a season without a shortage of audacious ideas, but in execution it's all over the place. Some episodes have all their ideas on point, with excellent results, like fan favourite "Sleepless in Ponyville", while others squander their potential, like "Games Ponies Play", or don't have a good idea to begin with, like the aforementioned "Spike At Your Service".
It's credit to the talent of the writers, then, that S3 is still pretty decent. High points like "Wonderbolts Academy" and "One Bad Apple" showcase a lot of character progression, while even lesser episodes like "Too Many Pinkie Pies" and "The Crystal Empire" showcase some good ideas. This is also a season which pushes the plot forward a lot, which has generated its own controversies. The first of these was Discord being "reformed" in "Keep Calm and Flutter On", an episode I find to be a high point of the season. This was an audacious move, as it takes the most popular villain of the series and reverses his role, and I think it paid off. The other controversial change is something that I'm sure you're all sick of by now: The season finale, Magical Mystery Cure, which had Twilight become an alicorn princess. This was an unprecedented happening, with the only real thing leading up to it being the framing device of the S3 premier. This has inspired a lot of debate, which only really cooled as people moved on. I've made my opinion clear. The fact is, though, that this season had long-lasting ramifications for the series, and this is something we have to deal with.
The most recent season, S4, is a bit of an opinion splitter. Consensus seems to place it above S3, which is understandable, but many seem to put it on a pedestal with S2. However, quite a few people I know lost interest over the course of the season, and that's where my opinion lies. Sure, the season started strong, with the first 12-episode run having a number of entertaining episodes sprinkled with a couple iffy ones, but even in this collection there's a distinct lack of something: character development. S4 attempts to compensate for this with a running plot, taking place over a series of episodes involving the mane six coming to terms with their elements. Sure, it's nice to see the characters getting back on track, but even though quite a few of these episodes were phenomenal, such as "Pinkie Pride", these were episodes that merely showed character progression that has already happened. Midway through the season, there was a slump where episodes never passed above being passable, which is what well and truly got me out of the season.
I think the high quantity of filler may be partially because this season introduced a ton of new writers, and consequently many episodes are first attempts. Considering this, we can probably expect better from S5, especially since some of these writers have interesting ideas. For example, "Testing, Testing, 1, 2, 3" took a different approach to the show, having a much more outlandish tone while also possessing an uncommonly ambitious lesson. Having the running story gave us a precedent for more big stories in further seasons, and also successfully reintroduced running themes for the first time since S1, if in a more limited form. This season also had the task of dealing with Twilight's new status, and initially it seemed it was gonna have an interesting take on it. However, quickly this fell to the wayside, with Twilight's status being heavily ignored, with attempts to address it lacking depth all the way up to the finale. Of course, "Twilight's Kingdom" is an excellent episode, serving as a nice bookend for the stretch of the series started in the second season. A lot was done in this finale, which gives S5 a lot of promise.
That's where we are now. S5 is right on the horizon, with a lot of potential. It's still MLP either way, so the season will unlikely be bad, but the writers now have a huge number of possibilities. Let's not get into a wish list quite yet, but with the mane six now given thrones in Twilight Sparkle's new castle, there's a lot of potential for a lot of new ideas and interesting changes. Who knows what we might see in S5? The possibilities are endless. I'm excited.