@Every comment here. Thank you for the new headcanons. :P

in MY headcannon, magic has a speed limit. like the speed of light or the speed of sound, there is the speed of magic in the my little pony universe.  
This is the reason that ponies aren’t teleporting everywhere or using magic to do everything.  
It would also explain the sonic rainboom, it doesn’t break the sound barrier, it breaks the magic barrier. Since magic augments a pegasus’ flying it would make since that pegasi cant surpass the speed of magic. Rainbow dash, however, can break the magic barrier since her wings are so powerful. with the combination of magic and wingpower, she can break the magic barrier resulting in a discharge of magic in the shape of it’s purest form: raw energy, such as light and sound from all over the electromagnetic spectrum (resulting in the rainbow of colors).
 
It’s just a basic theory i haven’t really put any thought into…

Each of the three pony races has strengths.  
Unicorns are obvious. Conscious control of their magic has a lot of utility, especially considering the show seems to imply that any unicorn can learn any spell if they are willing to but in study effort.  
Pegasi have flight (which might be magic assisted) and weather control, which means they have good domestic and military utility. Just ask anyone who tried to invade Russia at the wrong time of year.  
Earth ponies seem to get the short end of the stick, but they actually have a decent spread of abilities. They have more strength and stamina than the other two races. So in a military situation, properly trained Earth ponies are those soldiers that just won’t go down, no matter how much punishment the enemy throws at them. And once they get to the enemy lines, they’ll have the strength to more easily smash through their lines.  
On the domestic side, Earth ponies have a deep connection with the land and it’s creatures. This means that earth ponies are the best at growing and/or procuring food. Sure, they need pegasi or unicorns to provide the appropriate weather, but without the Earth ponies, the other two races would not be able to produce enough food to feed their populace and many ponies would starve. Also, barring oddities like Fluttershy, a druid-ish type with lots of animal friends is more of an Earth pony thing.

we all know the sea ponies would beat them all

@Karazor  
You don’t seem to get it…

HOW MANY TIMES DO I HAVE TO SAY IT?
 

 
NOPONY BUCKS WITH CLOUDSDALE
 
 
NO UNICORN CAN HANG WITH THIS  
 
PRECISION? YEA, WE GOT THAT TOO UNICORNS, YOU DON’T OWN THAT EITHER!!!
 
 
WHAT’S THAT? MAGIC FROM AN EQUIVALENT AMOUNT OF UNICORNS COULD MAKE MORE LIGHTNING?!? YEAH, BUCK DAT TOO, ZYUCKAS
 
No offense Zecora.
 
 
None taken.

It’s funny, super-strength would normally be a badass ability to have, but next to flight+weather control and outright spellcasting it starts looking a lot… well, weaker, pardon the pun.

We earth ponies are like Batman  

@Jackarunda  
Rainbow could probably blitz Twilight if she had to.
 
Organized pegasus military campaigns would be a terror. Ever use Storm Call in Skyrim? Plus total mobility and the ability to outright ignore terrain.
 
Earth ponies seem to get the short end of the stick. I figure they were probably the peasants that the unicorns and pegasi fought over.

Twilight is totally an OC.  
She’s the fanchar-come-reality of someone from X/1999

@Jackarunda
 
Well hey. The other ponies have had thousand or so years to get used to unicorns and their ‘freaky magic’
 
Would be amusing to see how it scares the crap out of non pony races though

Twilight’s abilities would still trump all that. My point stands.

Does it bother any of you that Applejack could kick another pony’s brains out without breaking a sweat? Or that any pegasus could drop any other pony from a great height and they’d just go splat on the ground??!
 
_{sheesh, you guys…}

Doesn’t it bother you though that it seems Twilight would be able to effortlessly eviscerate any of the other mane 6 in a mere moment?
 
Seriously, she lifted an ursa minor UP, along with a WATER TOWER full of milk and carried him MILES away. That kind of force application at a distance is ridiculous. She could tear a pony in half without a second thought.

Personally, I like Pegasi.
 
No magic to speak of aside from their natural affinity for the elements of weather. And considering their long past military culture they once had, that could have been a terror.
 
Think of the possibilities with an army that had the Japanese’s Mongol destroying Kamikaze on demand
 
And they seem to use their minds for engineering feats like the Weather factory, constructing almost as much as Earth ponies.
 
Of course, in my head canon they aren’t as strong and long lived and stable (lol) as earth ponies and lack the focus and devotion to knowledge and art that Unicorns typically have.
 
@Jackarunda
 
Hah! Using science with magic! Who do you think you are, Twil- waitaminute

Ah-hah! I figured unicorns were literally squishymages! I bet they can’t wear full armour without an encumbrance penalty either!
 
 
Also I kind of figured that it required more energy due to how it affects non-Twilight unicorns too. Twilight and her brother are just naturally pro-tier.

Personally I’m more concerned with the fact that unicorns seem grossly overpowered. I want to see some canon evidence that the three races are roughly equal.
 
Like, in my headcanon, using magic take an inordinate amount of energy. Like this: to lift an apple one meter takes one joule of energy plus 2 or 3 joules of waste heat caused by muscle friction and cellular activity. For a unicorn to lift an apple one meter with telekineses cost a total of 6 joules bodily energy, though telekinesis has more potential for usefulness than hands/arms (can act at indefinite distance, can act on indefinite mass or size, can act with practically indefinite precision). All magic is just less efficient. Either this, or unicorns have ‘magicka’ which is depleted with use and regenerates naturally. This limits their ability just like it limits the abilities of mages in Skyrim/Oblivion.
 
And I wanna believe that unicorns are exceptionally fragile. They’re not prone to athleticism, their bodies have low strength-to-weight ratios, and magic use requires concentration the likes of which is not available unless the unicorn is in total control of their state of mind. Only the most skilled of unicorns can maintain a magical grip on something when under duress.
 
The part where Sweetie Belle flicked Rarity’s horn to screw her up was good.
 
I also have in my headcanon that a unicorn’s horn needs to be exposed in order for them to use magic effectively. If a unicorn tries to do something intensive with magic while their horn is covered by something, they feel too stifled to make it work.
 
Twilight is the ELEMENT OF MAGIC, so she’s pretty much the most skilled unicorn in Equestria when it comes to magic. Her abilities do not accurately reflect those of a common unicorn.
 
Shining Armor was able to cast that protection spell due to his latent magical ability (related to Twily), and through tons of practice. Also he’s a BAMF. And the spell caused him to get horrible headaches. And it wasn’t perfect (broken through after not being renewed for a short time).
 
TL;DR, STFU JACKIE

Down with the filthy mud ponies! Unicorn power!

Fuck yeah, Abram Games.