With the exception of #1, the others are in no particular order. You'll note a very marked W/T bent to my selections. Pretty simple: I'm basically a W/T shipper.
1. Tempest Fugit – All three by LipKandy. Tempest Fugit S7 is my very favorite fan fic. It is post season six and Buffy and Willow are taken back to S4 by a spell. Willow has to deal with being suddenly returned to Tara and knowing that she's not where she belongs. The resolution of the fic is spectacular and magical.
Tempest Fugit S4 is also very good without any darkness floating around. Tempest Fugit AZ (in progress and a long time without an update) is very good too but I hate to recommend an unfinished and unupdated fic.
2. Unexpected Consequences – Long but very awesome. It deals in great detail with the consequences of Willow's memory spell on Tara and definitely makes Tara the hero of the piece. There is an unfinished sequel called Milestones.
3. The Master Willow Series (unfinished , BDSM) - This is my favorite vampire Willow story. Willow is a master vampire who returns to Sunnydale with her sire, Drusilla. She takes control of the town as well as of her pet, Tara. This story does a really fantastic job of communicating the complexities in the Master/servant dynamic.
4. Gods Served and Abandoned – This sequel to On Second Thought is one of the funniest pieces of fan fic ever.AntigoneUngone's dialog is my absolute favorite. The sequel to GSA, As Time Goes By, is in progress.
5. Hellebore by Chris Cook – This is the only uberfic on my list and with good reason. It's set in the AU of a video game? Seriously. What particularly impresses me with the fic is the level of planning Chris had to put into it for everything to tie up at the end. The sex is not bad either. Fantastic detail.
6. Common Areas – This is a fantastic story and the first one fromHiddenWatson (sometimes just Watson). Tara and Willow correspond via a magic mailbox even though Tara is in 2001 and Willow in 2004. When Willow learns of Tara's death, can she change history in spite of her vow not to?
7. Midnight Stretch –Verdant (BDSM) – In spite of it's brevity, I find this to be the most powerful BDSM or PWP piece I've read.
8. Paths Diverged/Divulged – JustSkipIt – Yes, I've just put one of my own fics on the list. It's my list. This fic is a sequel to W/T Season 3 Y'all. It takes post Tara's death and deals with Willow's plan to "acquire" a new version of her lover. As usual, she finds that everything doesn't exactly go according to her badly contrived plan.
9. The Amazing Kitten Race –HiddenWatson – Yes, her second entry on my list. No slight on GSA, but this is now the funniest ff I've ever read. Imagine that the characters we know and love (or not) are competing in the Amazing Race. Now imagine that you are reading the Television without Pity recaps of the episodes. It's an exercise in snarking and sometimes in snarking on snark. Most episodes include a W/T centered "extra."
10. I'm leaving this blank for all the many unfinished fics that I really love but won't recommend until they get some regular updates.