Civilised Conversations by Melissa begins on the eve of the big battle in "Chosen" when Xander comes down to the basement to ask a favor of Spike--he wants to go for a walk. Together they make a tour of Sunnydale and of Xander's life. They flirt, they kiss, they have the talk that we all wished they could've had in canon. A beautiful, bittersweet fic full of insight into the characters.
Melissa has also written two sequels to CC. The first is
And Afterwards Remember which goes AU early in AtS S5 when Xander is sent back in time to meet William in 1880 London to get some of William's blood for a re-corporealizing spell. They spend a lovely day together and, before they part, Xander tells William where to find him if he's ever in LA. In 2004 Spike re-corporealizes but, having forgotten Alexander, wonders why Xander's involved. Of course at the end Spike does fit the pieces together and goes to Xander or perhaps it's William going to Alexander.
The second is
A Harsh Master, dealing with Xander's having to learn how to to deal with an over-protective vampire. They are both excellent reads though not quite memorable enough to join my top favorites--I'm so horribly picky.