From VikingZX:

Flight Of The Alicorn

OFFICIAL DESCRIPTION:
Rarity's exploits in "Sweet and Elite" set in motion a chain of events that will find the unicorn dressmaker taking part in the greatest airship race in the world, the Alicorn's Cup. Excitement will give way to terror, as Rarity soon finds herself the victim of a plot against Equestria, and shipwrecked far from home with her least favorite stallion, the boorish Prince Blueblood. Rarity will learn of treachery in Canterlot and abroad, and will have search for something of substance beneath the prince's obnoxious veneer as she struggles to not only save herself, but her entire nation.

One of the best stories to come out of the entire fandom, quite simply. Excellent characters and characterization, excellent storytelling, a wonderful grasp of humor, adventure, seriousness, and plot. If you have not read this story, you've simply missed one of the greatest that the fandom has to offer. "Flight of the Alicorn" could quite easily be marketed as it's own standalone fantasy work and be a successful product.

Oooh, a quarter-million words. Good thing we ended up getting ten months to read them!

(I didn't even start on it. <_< )