I've often found that some of the most popular authors in the fandom aren't necessarily the best, but several of those already recced are indeed quite good.

In my opinion, one of the best is nwhepcat. There is nothing of hers I don't adore, and she makes me like characters I normally wouldn't. You could read anything of hers and see the brilliance, but I can't rec enough Wait for the Beep (http://www.echonyc.com/~stax/Buffy/nwhepcat/waitfor.html) (Xander, post-Chosen) and Lilac City (http://www.echonyc.com/~stax/Buffy/nwhepcat/lilac1.html) (Xander/Faith), which is probably the best post-Chosen fic I've ever read, bar none.

*smooch*