Whenever people ask this question, the fic that comes to mind is always that one Chani/Irulan longfic for the Dune books. I only read it the one time, but it was so good. Lots of chewy plot stuff plus slow-burn shippiness I could buy. And 26k!! Truly a Yuletide fic to aspire to.
http://archiveofourown.org/works/5426117