Unicorn Character Tiers · 9:27pm Jan 16th, 2016
Oh yes. I'm going there.
I actually wanted to sit down and see how easily I could sort unicorns into tiers based on the magic we see them perform and what we know of magic in the show.
Normal Unicorns
We should start by establishing a baseline. Normal unicorns are shown capable of telekinesis, with the implication from various sources that levitating multiple objects at once and levitating large objects is difficult, though not uncommon. As we'll see as we go on, many unicorns have the ability to produce light from their horns in some manner. However, it is also clearly said that most unicorns are only able to use a few elementary magic spells, with some having higher power levels related to their Cutie Marks. We also see some unicorns are able to project beams of magic from their horns, but the actual power of this varies: Twilight can blast a changeling away, but Trixie's blasts only tickle Rainbow Dash.
Your standard rank and file unicorn is a C tier, only able to perform the most basic of magic.
Twilight
It goes without saying that Twilight, as a unicorn, is the top pony here. She knows a wide variety of spells, is mentioned on several occasions to have a higher-than-normal aptitude for magic due to being the Element of Magic, and no other unicorn in the series has shown the same level of power as her. However, Trixie under the affects of the Alicorn Amulet was able to do an age spell, which Twilight says is only for the highest level unicorns; Twilight later attempts to do an age spell and only temporarily causes a flower to retreat in its bud. Thus, given that she cannot do age spells to full power but knows that it is possible to do, there must be a power level higher than pre-ascension Twilight but below alicorn level.
Twilight is a solid S tier.
Rarity
As cool as Rarity is, she is not very powerful in terms of magic. She has her special gem-locating spell, but this is her special talent as shown in the Cutie Mark Chronicles. Twilight is able to duplicate it, but then Twi is a special case; otherwise no other unicorn has attempted the gem spell, so we don't know if it's hard to do or not. The only spells she shows on a consistent basis are telekinesis and light projection in some of her fashion shows (we'll presume that the scenery shown in her Season 1 show were backdrops of dramatic effect). That said, she does show a great variety of skill in those spells, able to levitate and manipulate multiple objects at once and manipulate them very finely. We see her fire magic beams in Do Princesses Dream of Magic Sheep, but of course that's a dream.
I'm going to say Rarity is a C+ tier: she doesn't have much more magical potential than normal unicorns, but she shows good skill with the spells she does know.
King Sombra
Oh, this should require no explanation at all. Sombra's powers include negating the magic of other unicorns, creating compelling illusions, and is skilled with dark magic, implied to be a very powerful and dangerous skill. In "Twilight's Kingdom," Zecora says her potion needs alicorn magic to activate, and Twilight uses dark magic in response, implying mastery of dark magic is a very high-level skill. Now, in the comics, Sombra is not a real unicorn, and demonstrates the power to petrify other ponies, even alicorns. But the comics are Broad Strokes canon, so we don't know how strong Sombra really is.
Still, with the power he does display, Sombra is definitely an A+ tier.
Shining Armor
This is a tricky one. Shining Armor is able to use the same magical beam attack as Twilight, so we know he's pretty skilled. Shining is able to project powerful, prolonged shield spells, but this is implied to be his special talent: his Cutie Mark is a shield and he says in Canterlot Wedding that he's the only one that can sustain the shield protecting Canterlot, implying perhaps the Princesses can't. That said, while Sombra is able to negate Shining's magic with his crystals, Shining's shield is able to keep Sombra out, and even when he's out of it, it takes changelings a sustained period of time to break down his shield. Later, when he's supercharged by Cadance's love magic, the shield spell is erected in an instant and banishes the changelings miles away. In short, even if the shield spells he uses are his special talent, Shining Armor shows a great deal of power with them.
Shining Armor is an A tier.
Starlight Glimmer
I really didn't want to rank her, because I still find her power levels in The Cutie Re-mark too implausible. However, to put my biases aside, Starlight Glimmer is definitely a very powerful and skilled unicorn. She's able to levitate herself for prolonged periods of time, can levitate multiple objects at once, and fire magic beams. In terms of her cutie mark magic and her time travelling, it's difficult to quantify either of them. The former, we did see Star Swirl's spell change cutie marks in Magical Mystery Cure, but that was somehow related to the Elements of Harmony, so Starlight seems stronger than that. Starlight's time travel, meanwhile, somehow required the Cutie Map to use. Finally, we don't know what her special talent is, so it's impossible to say what power of hers she's shown is just really good magic, and which is her magical talent giving her an edge.
Starlight is at least an A+ tier, possibly S or even S+; we need to find out more about her and her powers to be sure either way, but the possibility is there.
Trixie
Trixie shows great skill in telekinesis. She also of course has her fireworks magic; while we don't know for sure how advanced a spell it is, she shows great skill with it, creating multiple blasts at once in a variety of forms and colors. Trixie turns Rainbow Dash's rainbow against her, summons a storm cloud and induces it to zap Rainbow, turns Rarity's hair green, and is able to produce visions of light and blasts of smoke. These all take a great degree of talent and are things we've never seen any other unicorn do (once again, Twilight is the exception). The weather manipulation in particular is impressive. Later in Magic Duel we see her produce a magic blast from her horn, but it only tickles Rainbow Dash.
Now, one point with Trixie is that she is a magician. Magicians rely on trickery, deception, and sleight-of-hand. So it's tricky to say how much of what she does is really her. She might stir up Rainbow Dash with her own rainbow, or perhaps that's her magic doing the spinning and she makes it look like it's the rainbow. She might produce a blast of smoke from her hooves, or, as in my fanfiction, perhaps she just uses smoke bombs and conceals them from view until she lets one off. However, in terms of tier rankings as defined here, it doesn't matter - even if Trixie lacks the raw power to do all her tricks for real, she makes it look real, which still takes skill and creativity.
Trixie is definitely a B tier, a touch above normal unicorns. Was there ever any doubt? For the sake of ranking, I'll also note that with the Alicorn Amulet's definitely a S+, able to outdo the pre-ascension Twilight with ease.
Sunset Shimmer
While I adore Best Human, her power levels are difficult to quantify. Any time she uses magic as a human is out for a multitude of obvious reasons. As a pony, she's able to teleport, implied to be a high-ranking spell that no other unicorn does save for Twilight. Sunset is also mentioned to be Celestia's student, implying a great deal of magical potential. In the comics she projects a blast of fire from her horn, but comic canonicity aside, we know that Sunset's Cutie Mark is a sun, so she may just be skilled with fire magic. So all we have to go on for sure is teleportation and the implications of being Celestia's student. But that's enough.
Sunset is at least an A. It's possible she could be higher, but we don't see enough of her power to be sure.
Moondancer
From her studying, one could presume Moondancer has an advanced understanding of magic, but we don't know how much of what she's read she's able to actually do. She's able to levitate and move Twilight, a full-grown pony, and is able to burst Twilight's bubble of silence; however, it's likely Twilight didn't put much energy into stopping either of these, given Moondancer was a friend. She otherwise only shows telekinesis, though able to levitate multiple objects for sustained periods of time. Twilight implies she'll teach Moondancer the spell to enter books, but we don't know if Moondancer is actually strong enough to use it.
I'm gonna say Moondancer is a C+.
Flim and Flam
The Flim Flam Brothers are definitely a low-tier. Their only showed ability is telekinesis, and they usually only manipulate one object at a time. A point of contention here is the Super Speedy Cider Squeezy machine, which they power with their magic. However, it's never explained or shown how they power it, they fire beams of magic from their horn that trigger the machine. The machine may just run on a battery they recharge with their magic, or their magic may take control of the machine to autonomously move the parts.
I declare them a C+ class, barring a convincing argument to bump them up. I'd say they're merely a C, but the magic beam stuff is something we don't see often.
Sassy Saddles
She is a C+ tier, as like Rarity she has light projection magic and can manipulate several objects at once with her telekinesis.
Royal guards
The Canterlot guard do some basic things. They project beams of light from their horns for prolonged periods of time. One of them is able to unlock the Star Swirl gate with his magic, implying a dexterous use of telekinesis. I don't think it's unfair to extrapolate that all the guards can do that, since this seems a basic job skill they would need. In Equestria Games, we see magic-negation portals being maintained by the Royal Guards, and it is later implied they can negate the spell en masse. While it's uncertain if technology is augmenting their skills to do this and how much if so, and if it's a group effort or not, this is still an impressive thing.
The royal guards are a B tier, possibly higher or lower if it's ever clarified how their magic negation in Equestria Games worked.
Thus, the final tier list is as follows.
S+
Trixie (with the Alicorn Amulet)
S
Twilight Sparkle
A+
King Sombra
Starlight Glimmer (possibly a S or S+ class)
A
Shining Armor
Sunset Shimmer
B
Trixie
Royal guards
C+
Rarity
Moondancer
Flim and Flam
Sassy Saddles
C
Normal unicorns
I now eagerly await the many arguments in the comments about how I'm wrong. 
Where does this tier-coding system originate from? I've seen it multiple times at different lists I but can't really place it. Are these 4 types (and their "+" counterparts) the only tiers or are there any in-between or lower?
As for the list, I mostly agree with it. Though I have the feeling Sunset could be higher on the list - I don't think Celestia would take anypony for apprenticeship below tier S.
Well, traditionally tiers are alphabetical, it's just how it goes usually.
Heh, your tiering system reminds me of video games. S rank is impossible to get!
Anyway, mostly agree with your assessment. It's very hard to really know what makes a unicorn powerful. Is it the amount of spells that they know, or the power in which they use them? Can unicorns learn spells just by reading about them in a book, or does the unicorn have to have a particular talent to do so?
For pegasus ponies, it's pretty easy. The faster they go, the better. Heck, even Twilight developed a device to determine pegasus wing power.
Earth ponies... eh... do stuff? Again, one of those things that never really gets explained. I think it's because the writers find them boring and give all the flashy stuff to the magic users.
So yeah, nice list. It's too bad most of our knowledge comes from unicorns that break the mold. We have Rarity, but her character isn't about powerful magic, so it's hard to draw conclusions there.
Headcanon away!
I like a ranking system, but I feel like just letters only works for Earth ponies. There magic hasn’t been shown to be as complex as either Pegasus or Unicorn magic. For unicorns in particular I think there abilities could be ranked like they are in the Naruto expanded universe materials.
Something like this.
vignette2.wikia.nocookie.net/narutofanon/images/f/f6/Stats_Chart_-_Taiki.png/revision/latest?cb=20110723202717
Maybe not this complex though. For unicorns I would have a square with Raw Power, Finesse, Knowledge, and Sustainability*.
* Basically time spent being able to sustain a spell or the magic aura.
I think your ranking system holds up pretty well. I'm willing to declare Starlight S+ now, but I see the wisdom in holding off until the season 6 opener, until we will see more. One thing that you avoided ,that may piss off a lot of people, is putting alicorns in the tiers. I know some people will say every alicorn is several magnitudes more powerful than any unicorn, but we also know that Sombra was able to fight off Celestia in a massive war, despite virtually all his soldiers being earth ponies, so he must be roughly her power level. Maybe that means Sombra is also S+ power level, maybe it means Celestia is only an A level.
I don't really think that you are wrong in regards to measuring their overall power.
I think however that these are irrelevant in the grander scheme of a story.
Any magic user can be defeated provided you use the power you have correctly.
While true, we never actually see Sombra fight Celestia one-on-one, so it's hard to say.
On the other hand, the combined might of four alicorns in Twilight is able to match Tirek, who is charged up with the magic of dozens (at least) of normal ponies as well as Discord. So alicorns must indeed be powerful. Of course, there's also the presumption all alicorns are equally powerful, which is unknown - Celestia and Luna are special, and Twilight is the Element of Magic while Cadance is not.
For the record, I was jokingly going to rank Discord as Z-tier, but nah.