Pairing: Buffy Summers/Spike
Fic Title: The Worst Journey in the World
Author: shapinglight (Glassdarkly)
Link: http://distortingmirrors.notanothersite.com/theworstjourneyintheworld.html (whole story)
Rating/Warning(s): R for violence and swearing
Why This Must Be Read: shapinglight is better known for her slash and kink fic, but she has also written many long and plotty Spike/Buffy fics and this is my favourite of them. She writes, IMO, the best Buffy in fandom - strong yet flawed, but most importantly a hero - and is a rare writer who seems to live both Buffy and Spike equally.
In this story, set five years after "Not fade Away" (no comics canon), Buffy and Spike set out on a dangerous journey across the frozen country to save the world together with the help of Wesley, and the result is 25 000+ words of good plotting and excellent characterization.