Yes, it's the return of the "Best Pony" series, in which I write articles about why various characters in the show, selected by Kickstarter donors, are Best Pony.
Sorry about the last-minute switch from doing an episode essay. Busy with book-related stuff. Next weekend we'll be back to episodes.
Also, sorry for lack of responses to comments in the past week. I can no longer access the front end of the site from work, Blogger does not allow commenting from the back end, and trying to use my phone gets the comments eaten more often than not. I have now gone back and commented on every comment in the past week to which I have a response.
As I have stated repeatedly in this series, the definition of "best" is generally quite fluid and flexible. However, if we are to consider whether the show itself treats a character as being "best," a fairly solid answer emerges. There is one pony who is consistently regarded with respect, love, and awe by all non-villainous characters, who represents the pinnacle of power in every sense of the word, and who is also held up as an aspirational figure and role model for the closest thing the series has for a main character.
That pony is, of course, Princess Celestia. However, even if she weren't named in the title of this essay, she is still quite recognizable from the description in the previous paragraph. Only Luna rivals her in terms of power, and even there Celestia is the more significant and powerful figure within the narrative. Only Pinkie Pie comes close in terms of being universally loved by non-villains, and even there she sometimes gets on other characters' nerves, such as Rainbow Dash in "Griffin the Brush-Of" and Applejack in "The Last Roundup."
But it is Celestia who commands universal respect and adoration, as we see in multiple episodes right from the start of the series. In the premiere, "Friendship Is Magic Part One," her disappearance is treated with a stronger negative reaction by the crowd, an audible gasp, than Nightmare Moon's appearance. An ancient evil is bad, yes, but for Princess Celestia to not appear when she promised? That is apparently far more shocking. Later, in "Friendship Is Magic Part Two," we see that there is no question that she has the authority to order a pony to uproot her life and settle in a new town, though admittedly this is something the pony in question, Twilight Sparkle, very much wants. Later, in both "Swarm of the Century" and "A Bird in the Hoof," her arrival in Ponyville is treated as a festival occasion, with decorations and special dinners.
However, she also commands significant awe. Nowhere is this clearer than in her appearance at the end of Season Two's "Lesson Zero," when her stern shout of "Twilight Sparkle!" immediately cows all of the Mane Six. Part of this awe, of course, is due to her sheer power. In "Lesson Zero," she's able to wipe out the effects of Twilight's out-of-control magic in an instant. In "A Canterlot Wedding," she is nearly able to single-handedly defeat Queen Chrysalis in a straight contest of power, at a time when Crysalis is gorged on the love energy generated by a relationship involving a pony whose special talent is literally love. In "Princess Twilight Sparkle," we see her go toe to toe with Nightmare Moon and hold her own, though she ultimately needs the Elements of Harmony to defeat her, and in "Twilight's Kingdom," the combined power of the four princesses--of whom Celestia is consistently depicted as the most powerful--is shown as being on par with Tirek wielding the combined power of the entire rest of the population of Equestria plus Discord.
But what makes Celestia best pony is not just power, it's the wisdom to use that power properly. The show repeatedly depicts and contrasts good and evil (or at least deeply flawed) versions of the same kind of power, such as the economic power of the Apples as opposed to the economic power of Diamond Tiara or the Flim-Flams, the speed and energy of Rainbow Dash as opposed to the speed and energy of Lightning Dust, or the love magic of Cadance as opposed to the love magic of Chrysalis. The consistent pattern is that good power is characterized by restraint and empathy. The Apples, despite being founders of the Ponyville and having control of two of its main products, live frugally and humbly, as opposed to Diamond Tiara's power trips or the Flim-Flam Brothers' grasping avarice. Rainbow Dash holds back to avoid risking harm to others, while Lightning Dust charges ahead recklessly. Cadance helps a fighting couple remember that they care about each other or works with Shining Armor to create magical shields against evil, while Chrysalis seeks to control and consume.
Within the show, Celestia is the paragon of this restraint and empathy. Right from the start, at the end of the premiere, she recognizes what Twilight Sparkle really wants and uses her authority to provide it. In "Lesson Zero," she recognizes that the underlying problem is that Twilight has advanced to a new level in her studies, and changes the rules for friendship letters accordingly. In "The Crystal Empire," she recognizes that it's time for Twilight to be truly tested, and so sends her rather than (quite easily, one imagines) taking on Sombra herself. But perhaps the best example is in "The Return of Harmony," when rather than using her power to fight Discord directly or even to break the enchantments on Twilight's friends, she recognizes that Twilight has already written the spell that will free them, and sends it back to her through Spike.
For all that the fandom jokes about Celestia's political authority and her occasional hints of the trickster mentor, she really does exemplify this quality of power restrained by empathy, which is most likely why she is such an important mentor figure for Twilight. The show makes quite clear that Celestia is Twilight's role model, and has been for at least her entire adult life: the moment at which Twilight enters young adulthood is also the moment at which she becomes Celestia's protege in "The Cutie Mark Chronicles." Twilight's most significant panics are at moments when she fears disappointing Celestia, most obviously in "Lesson Zero," but also notably in "Swarm of the Century" and "The Crystal Empire."
Recall that the series began, effectively, as the story of Twilight Sparkle learning to relate to other ponies. The only significant relationships she is shown to have at the start of the series are Spike and Celestia, and of the two it is Celestia from whom Twilight is explicitly learning. The implication is thus that Celestia possesses the knowledge that Twilight is seeking. Within the context of a show about learning, can there be any higher position than the teacher, the character who already knows the lesson?
Even as the series has expanded in its scope, so that it is all of the Mane Six rather than just Twilight learning and developing, they continue reporting their findings to Celestia throughout Season Two and Season Three, continuing her role as the "grader" of the friendship lessons, with the implication that she has already mastered them. Then in Season Three as Twilight begins to develop into a Princess, it is Celestia who takes the lead in administering her tests at the beginning and end of the season, and ultimately Celestia who appears to Twilight in her death-vision and explains to her that she has moved beyond Celestia's capacity to teach. Even in raising Twilight to her own level, Celestia's great wisdom is revealed: she has been watching Twilight since the beginning, as she tells us in song, which means she saw this potential in Twilight from the start. Her empathy is so great that she is able to realize capacities in Twilight that Twilight herself couldn't see.
The living epitome of power tempered by wisdom. The role model. The universally adored leader. Celestia is all of these things, and yet at the same time is able to crack a joke, pull a prank, even be bored at the Gala in "Best Night Ever." She is no goddess, no sorceress in an ivory tower; she is a pony, and as a pony, she is best pony.