It's great to see a lot of fellow Rarity lovers on this thread, but for some reason, the people supporting RD aren't as...how do I say this. They're not giving logical reasons for why they like Rainbow Dash. They just say something like "She's awesome!" or "Meh. Rarity needs to be 20% cooler."
(By the way, I don't know how that phrase got so popular. Dashie happened to be being a jerk at the time, and I never saw the appeal.)
So to play a bit of devil's advocate, I'm going to take a few seconds and stick up for RD. I voted for Rarity in the poll, because she is my favorite pony and will (hopefully) stay that way, unless the writers do a 180 and botch her spectacularly.
Rainbow Dash, like Rarity, is often written off as an athletic jock who could potentially care little for other ponies. We see clearly that she appears not to care what other ponies think of her, what with her flyaway mane and hatred of frills and frou-frou nonsense. But she does care for the approval of other ponies, which is what I think is the core of her character.
On the surface level, Rainbow Dash is quite annoying, and almost stereotypical; tomboyish girl who's athletic and thinks she's the greatest person in the world. But scrape a little bit beneath that and you'll discover a much more insecure pony than you'd think. In many ways, Rainbow is the most insecure of the Mane Six (Fluttershy notwithstanding.) She's terrified of messing up in front of people, and takes great pride in her racing and weather management abilities. She's also very concerned about not being seen as an "egghead" or, in other words, a geek. Her reputation means the world to her.
This is potentially disastrous for others around her. Rainbow could very well be a bully in a parallel universe, because beneath the jock, "King of Anything" attitude, there is a very worried pony who relies heavily on her skills in flying. Have we seen her brag about anything other than flying, excepting her broad claims of being "awesome" or "the best"? I don't think so. I believe (and this may very well be headcanon) that deep down, Rainbow Dash thinks flying is the only thing she is good at - damned good at it. (Pardon my Fancy.) So the need to constantly assert and assure herself that she's better than anyone else is really her way of soothing the inner insecurity.
Now let's take her relationship with Fluttershy. As I've said before, in a parallel universe Rainbow Dash could easily be an antagonist or villain; but she chooses to befriend and stick up for one of the weakest, shyest ponies in Equestria. Why is that? Is it because that Rainbow Dash somehow relates to her, on a different level than she can even understand? I don't think RD would admit that she's similar to Fluttershy if her life depended on it. Her relationship with Fluttershy also ties wonderfully into her Element of Harmony: Loyalty. Despite having an excellent relationship with the little yellow pony, Rainbow Dash clearly sees Fluttershy's faults, and for the most part, brings her up on it. She lets her friend know very clearly that she's veritably useless as a cheering squad, and would be a hindrance going up the mountain to face a dragon.
So I suppose the question that must be asked - is Rainbow Dash friends with Fluttershy because she sees a mirror image of herself buried deep inside? Or because she feels the need to protect her, and be a physical shield if necessary against bullies?
You could argue I'm dissecting their relationship too much, but frankly if Rainbow Dash wasn't friends with Fluttershy, her character would be extremely bland. Her relationship with Fluttershy really is the most redeeming quality of her - to show the audience that even an athletic, popular jock can be friends with the most shy wallflowers.
Yeah, yeah, I know. I'm rambling. Dx