So I found this image on Derpybooru.

The question I pose is, which of the Ravnica guilds would the mane six join?

Here are my picks:

Twilight Sparkle: Azorius.

Like the Azorius Senate, Twilight requires order, even at the expense of the freedoms of others.

Rainbow Dash: Boros.

Rainbow Dash's brash behavior and fierce loyalty would be highly sought after throughout the ranks.

Pinkie Pie: Rakdos.

Pinkie Pie would fit it with Rakdos, because she wants everyone to have a good time... whether they want to or not.

Fluttershy: Selesnya.

Fluttershy just wants to be surrounded by lots of animal friends.

Rarity: Orzhov.

Both the Orzhov and Rarity seem to highly value wealth and influence.

Applejack: Gruul.

Applejack was the hardest to pick. In the end, I went with Gruul. Applejack does have a stubborn streak and a strong will which would be welcome amongst the Gruul. Applejack's honesty wouldn't be out of place either, as the Gruul have no use for deception or cunning.

I actually made a blog post asking and answering this question. I'll recapitulate my theories here:

Twilight: Izzet. Passionate thinker. Consumes information ravenously across a wide gamut of topics. Researches and experiments obsessively, moving from one topic to the next as her interest shifts. Fluctuates between inflexible and unstable mindsets.
Rainbow Dash: Boros. Passionate, aggressive, volatile, charismatic, and fanatically loyal. Subtle as a brick. Highly goal-oriented.
Pinkie Pie: Rakdos. Hedonistic, carefree, parties like there’s no tomorrow (and occasionally makes it less likely there will be.) Completely unbound and unbindable by rules or restrictions.
Fluttershy: Selesnya. Cares for a vast number of creatures of all shapes, sizes, and diets. Gentle, but defends others ferociously.
Rarity: Azorius. Finicky, detail-oriented, controlling, contemplative, and organized. Good intentions, but not necessarily the right priorities.
Applejack: Golgari. Understands, appreciates, and uses the natural cycle. Advances own ambitions by helping the greater community. Unrelentingly stubborn.

The choices made in the fanfic City of Guilds were pretty good. (Twilight Sparkle Izzet, Rarity Azorius, Rainbow Dash Simic, Fluttershy Selesnya, Applejack Golgari, Pinkie Pie Rakdos, Scootaloo Gruul, Trixie Orzhov, Shining Armor Boros.)

My thoughts:

Twilight Sparkle: Mad scientist. Clearly Izzet.

Pinkie Pie. Fun loving. I see her as Rakdos.

Fluttershy: Speaks for nature in its natural form. Clearly Selesnya.

Rarity: Whichever guild has the nicest outfits. I can't see the Element of Generosity being part of the Orzhov. Azorius is a better fit than the others.

Applejack: Also close to nature, but she'd feel out of place in a plane that's all city. Selesnya is the best fit. She's a little too much into cultivating to be Gruul.

Rainbow Dash: I'll go with your choice of Boros, but were I to create a card for her, I think I'd make her black, since she's fairly self-centered.

Queen Chrysalis: She's secretly in charge of the Dimir.

And since you posted that Rarity picture:

Now, which shards of Alara would feel like home to each of the Mane 6?

Twilight Sparkle: Esper. Just think of the order!
Pinkie Pie: Grixis. She's clearly an abomination.
Rarity: Bant. That fabulous armor!
Applejack: Bant. Very honorable folk.
Fluttershy: Naya. She should be the Anima.
Rainbow Dash: Jund. Only the fit survive, and she's the best.

Most Killer combo:

+

But seriously, I'd play Selesnya, as an Izzet Leaguer, if I could play her:

The set is by Sorden on dA.

648748
You mean like this?

Oh, and has everypony forgotten about AJ Fluttershy possibly being Simic Combine?

The Simic Combine is Ravnica’s steward of nature and the wild; its mission is to preserve and advance the natural world even as Ravnica’s cities continue to grow.

648807 Here's my take on Rainbow Dash as a Magic card:

Rainbow Dash – 2BB
Legendary Creature – Pegasus Scout
Flying, Haste, Friendship 2
Rainbow Dash deals combat damage at the end of the declare blockers step, not during any combat damage step.
“I ALWAYS win.”
3/3

648819
The problem is that the Simic interpret "preserve and advance" as "mutate and hybridize." They're the relatively calm and collected mad scientists, the Dr. Moreau to the Izzet Doc Brown.
Selesnya, meanwhile, is about bringing all of nature into a single harmonious unity, whether it likes it or not. Given the choice between mad biology and war hippies, I think Fluttershy would opt for the latter.

648977
Well, I think it'd be a matter of who she got to, or got to her, first. The Simic would give her the elevator pitch to get her to the Combine, but from there, all bets are off. Now, if she were to run into both at similar times, then yes, Selesnya would be her choice. Eventually.

648706 Great minds think alike clearly, as most of those are identical to my choices in City of Guilds. I threw Rainbow Dash into the Simic because she's always looking to improve herself. And the Simic like to force evolution along.

649296
If we're going with fan interpretations for background characters, than Lyra would definitely be Simic.

"Fuse me with some human DNA so I can have fingers, please."

653000 Quite true, then again if Gatecrash is canon than the Sea Ponies would also belong there.

648706
Very good choices. Your picks for Twilight with the Izzet and Applejack with the Golgari were my second choices for those respective characters. I could easily consider changing my picks for Twilight and Applejack to your suggestions.

The only place I draw the line is with Rarity. Rarity's actions throughout the series scream "Orzhov" to me.

"Most Ravnicans see the Orzhov for the corrupt organization they are, but many are lured in by their promises of wealth, prestige, and longevity."

Rarity isn't afraid to make others feel guilty, even when she's being selfish.

Rarity has concocted a thick web of lies in order to secure power and influence amongst the wealthy elite.

She's manipulated people on multiple occasions to cajole them into giving her what she wants.

Her inability to control her greed has repeatedly gotten her in trouble.

It takes a special kind of opportunistic criminal mastermind to go from being a kidnapped victim to robbing your kidnappers blind.

For these reasons, I strongly feel that Rarity is most at home amongst the wealthy ranks of the Orzhov.

653079 I admit, you make a very persuasive argument. I may have to reconsider this...

Generosity to one's friends is easy when one regards them as an extension of one's self. Indeed, Black mana might make that kind of generosity particularly easy: among other things, it's the color of high-risk bargains, and it gave us "the only life point that matters is the last". Whether or not Rarity would be comfortable among the Orzhov, the case for her matching their colors is very strong.

648773 Twilight Sparkle might be most comfortable on Esper to begin with, but Esper won't be comfortable with her, and the stress of that would eventually drive her away. Despite Red being just as foreign to Bant, they strike me as being more tolerant than Esper of the loud bangs and odd smells that begin to appear wherever Madgirl Twilight settles down.
EDIT: And Pinkie Pie is a peer to Nicol goddamn Bolas. Of course she'd be most comfortable on Grixis. But she'll probably spend her time helping out the Vitals when she's not forcing the zombies to act out the music video to Thriller.

891601 Pinkie Jackson is a glorious mental image, and I thank you for it. Especially since I can't help but imagine Bolas filling in for Vincent Price. After all, who would be better qualified to speak of the funk of forty thousand years?

Oh, and if we're discussing shards...

Twilight Sparkle: Esper. They'd be very tolerant of her experiments if she told them she was looking for the secrets of etherium. After disrupting and repairing destiny, how hard could it be? Besides, Test of Metal showed us the power of befriending the Hegemon. If Sharuum were to make Twilight her right-paw pony, who would dare gainsay her?
Applejack: Either Naya or Bant. She'd either be aiding the sunseeders, tending the orchards of Valeron, or, during the Conflux, defending crops from vagrant plowbeasts.
Fluttershy: Naya. So. Many. Wonders.
Rainbow Dash: Jund. "That rainbow one kicked me" would become a common refrain throughout the shard. Goblins wouldn't know whether to revere her or despise her. (Alternately, on Naya, she could become the next matca superstar.)
Rarity: Bant has chivalry and an idealized aristocracy going for it, but Esper has those fascinating filigree aesthetics and such lovely weather patterns. How's a girl supposed to decide?
Pinkie Pie: She wouldn't be able to choose. Bant is the best place to hold parties, Esper has the best favors, Grixis provides the entertainment, Jund the pyrotechnics, and Naya the refreshments. And when the five collide, she can have a "Congratulations on the Restoration of Your Plane" party in the Maelstrom!

648672
648706
While these theories are all well and good, we have all forgotten the most important characters beyond the Mane 6. The Cutie Mark Crusaders! Here are my thoughts on the matter, both guild and shard wise.

Apple Bloom: Selesnya. She puts a great deal of care and respect into anything she does on or around her family's farm. She also seems to respect life in all its forms, though outliers do exist (DT and SS.) She values the opinions of her grandmother, brother, sister and sister's friends rather highly. For these reasons, I also believe she would be the most comfortable on Naya.
Scootaloo: Gruul. She seems very inclined to, given good reason, or any reason really, get all 'smashy-smashy'. She also appears to value her 'gut-instinct' and can be very impulsive. These are also the reasons that I believe she would feel most at home on Jund.
Sweetie Belle: Azorius. Her intentions may be pure, but she has an unfortunate tendency to focus on the unecessary and frivolous. She also seems to have a very strict moral compass and thinks many things through with a sensible logic, though these things have been known to, in collaboration with the other Crusaders, get her and sometimes her friends into trouble. Hence, the shard she would most fit into would be Bant.

2810073
Intriguing. At the time, I didn't consider the Crusaders on the basis of "too young to choose." After all, if you haven't figured what your destiny is yet, how can you decide which world-governing organization to join? Still, if I had to choose...

I actually see Apple Bloom in the Izzet, believe it or not. In any episode where she's more than window dressing, she's the outlier, the catalyst, the one Apple who wants little to nothing to do with tradition. She questions, she experiments, and she has no patience. Add in an obvious talent in engineering, and you have a mad artificer in the making.
As such, while she might find Naya or Bant the most comfortable shard, she'd likely find Esper the most rewarding shard. Provided, of course, she could evade the oppressive authority of the Hegemony. On both Ravnica and Alara, Twilight's patronage would help immensely.

I agree with Scootagruul; I doubt even Rainbow Dash could get her to deal with the discipline of the Legion, and the Cult of Rakdos is a bit too free with the pig's blood and casual maiming for her tastes.
In Jund, meanwhile, she would be Dash's sidekick/adopted little sister/surrogate daughter/symbolic link to equinity, making sure she who fights monsters didn't become one. Not that Scoot would be a pushover herself. I have this wonderful mental image of her on a goblin's back, sinking her teeth into its ear as it screams and flails and regrets having just completed the penultimate step of the dragon-calling ritual.

Sweetie Belle, meanwhile, I actually see in Selesnya. Two words: the Worldsong. She could go full Disney princess, singing to a massive audience of nonjudgemental forest friends. And anyone who interrupted her would face an army of disgruntled squirrels. Besides, she'd probably break something, run afoul of some obscure statute, set something on fire, or do all three at once within minutes of setting hoof in New Prahv.
Bant is definitely the shard for her. The others are bit too dangerous for the Crusader with the most common sense (not that that's saying much.)