One of my hands down favorite S/X writers is tabaqui.
I'd suggest for first timers: Little Runaway (and part II) (NC17)
Summary: it's a sweet yet at times a little dark work that asks the question - what if Spike and Dawn had hatched a plan to get out of town while Glory's in it, and had taken Xander with them for insurance?
or, one of my other favs: "Letters" (NC17)
Summary: Post Chosen, set in S5 of ATS where no one knows what to do with Spike... and both he and Xander go off to find themselves.
And then there's the fiendishly hot "The Education Series" by Pet (NC17)
Summary: Oz & Devon have moved back to down and Xander starts hanging out with them until he realizes who he really wants... (and that's where it starts). Freaky hot stuff, with a little drama thrown in the middle for good measure.
Can't forget reremouse's work, and I'd suggest "Doughnut Boy" (NC17)
Summary: A little spin on the wish-fairy, except this time she's a Justice demon and Xander has never really been just the doughnut boy.
finally (although with s/x there really is nothing *final*), I'd suggest savoytruffle's "Waking Up" (NC17ish)
Summary: S4 Xander comes back to town after the road trip & wakes up a demon, is caught by the Initiative and stuck in a cell next to Spike.
Anybody else want to add to this list? What are your fav s/x stories?