Difficult question since I read mostly Spike-centric fic. I was going to rec Rachel Anton's "Geek the Girl" but that's Willow/Spike.

I would suggest going to the BBF archive. They're very discerning about what they include there. You can do a search by many different parameters, including character (ensemble as well).

Here's a link: http://s8219.net/bbfarchive/cgi-bin/search.cgi