I think that Rainbow Dash and Rarity seem to get along pretty well, and they have quite a few things in common.They're both ambitious and confident, they both like to be the center of attention, they both care about their appearance in some way; while Rainbow may be a tomboy, she doesn't seem to mind wearing dresses and looking pretty every now and then. And Rarity seems to enjoy watching the Wonderbolts, judging from Suite and Elite.
And I've noticed some instances in the show that implies that they're pretty good friends (other than being apart of the Mane Six):
- Rainbow modeling for Rarity in Swarm of the Century, even though she didn't like sitting still.
- In Sonic Rainboom, Rarity was the only one to notice RD's stage fright and suggested that they must attend the Competition, and then Rainbow saves her life in the end.
- In Dragon Quest, Rainbow and Rarity were asking Twilight to join the two of them for lunch (or maybe it was breakfast, don't remember at the moment.), so I wonder if that's something they do often.
- And in Just for Sidekicks, Rarity and Rainbow were hanging out by the well with their pets, and the fact that Opal and Tank seem to like each other.
I think there might be other instances, but those are the ones that come to mind at the moment. I'd like to think that they were already friends before Twilight came to Ponyville.
They're my two top favorite ponies, Rainbow being first, and Rarity second. So, I hope there will be an episode about the two of them in Season 4.