A Model Romance by Sweetprincipale.
Summary: Set in early S. 6, a reluctant Spike and Buffy head out of town to investigate mysterious activities going on at a modeling agency where six disappearances have been reported. Forced to work together when they have no one else to help, a romance develops. (Rated M).

You can find it on FanFiction, and it's one of my favourite Spuffy stories of all time. I adore her writing! This story is sweet, angsty, smutty...a little bit of everything! I found the characterization to be unreal, and I felt as if was 'believable', like something that could have been an episode, but better.There is also a sequel, and she's written many other fantastic Spuffy stories (Although I haven't really been able to get into the more recent ones. But that one? I had to make an account so I could rave about how much I loved it).

Older and anything else by Nautibitz.
http://www.nautibitz.com/fic/older/index.html

Summary: A novel-length super-AU (everyone's human) story in which Spike is Buffy's new stepfather and they can't help but have a torrid, sekrit affair and fall crazy in love.

Well, since West of the Moon, East of the Sun has already been mentioned, I'm going to have to go with Fix You by Enigmaticblue. My bookmark is to Nocturnal Light.