I didn't used to be that interested in MLP fics, but then I read The Enchanted Library by Monochromatic
Everypony enjoys myths and ponytales, even if they know such things aren't real. Alicorns fighting against a spirit of chaos? An ancient princess trapped in a library under a tree, waiting to be found? Quite enchanting and fantastic tales yes, but nonetheless as fictional as Daring Do and other such stories. At least, that's what Rarity used to think.
She doesn't anymore.
This is an alternate universe story in a world where the princesses are all thought of as children's fairy tales. One day, Rarity comes across an ancient library, where she finds Princess Twilight Sparkle as a spirit, unable to leave. Rarity thus goes on a quest for lost books to return to the library and eventually free her.
Rarity is presented extremely well in this story (and most of Monochromatic's fics), and that makes it worth checking out on its own. But adding on that is her relationship with Twilight. The princess starts out very cold and distant, reflecting her thousand years of solitude and hopelessness, but slowly--very slowly--she warms up to Rarity in increasingly adorable fits and starts, until it blossoms into a full blown romance. I wasn't particularly interested in pony shipping, but this and Mono's other fics made RariTwi my OTP. Perhaps more important is that this romance doesn't overshadow the adventure aspect, and the adventure doesn't overshadow the romance. More than most other stories I've read, it balances the two really well, and makes both important.
It's a shame that the sequel, The Enchanted Kingdom, is right there on the sidebar with a synopsis that spoils how The Enchanted Library ends, because it goes in some unexpected directions that makes you doubt things will turn out well. Especially if you've read some of Mono's other works, which don't pull their punches on the drama.