Many people on the web keep very well-updated, wide-ranging rec pages; I doubt I'll ever be so thorough and comprehensive myself. Many fine authors and stories I've read aren't included here, but this is a sampling of some of the people whose work I've really considered outstanding over the years:
CazQ -- If there's a more haunting, beautiful, poetic AU out there than her XF story "The Furious Winter," I haven't read it yet. Phrases from this fic still linger in my mind years and years after I first read it; bet it happens to you, too.
Dianora -- Not only is she the Beta Reader to the Stars, not only was she a very cool roommate for one long, hot summer in Jersey City, but Dianora is also a fabulous writer. "Shot Through The Heart" is an inventive XF/Cupid crossover. (Remember "Cupid"? If you were one of those who didn't see the show -- ie, unfortunately, the vast majority of the viewing public -- you truly missed out but can catch up a little through this.) And she's the author of one of the few nonfiction recs on this page -- the key shipper resource, "Why the Moonlighting Argument is a Fallacy". Take this out for ammunition the next time somebody tries to tell you the fulfillment of your favorite couple would "ruin the show."
Emu -- The Emu has written beautiful stories for a number of different fandoms, but it was her TPM story "Chastity" that first made me realize her talent and her ability to follow through with a genuinely original and challenging concept.
Gyrus -- A fanfic stereotype says that male writers give more attention to the action scenes, but sometimes skimp on the emotional context. Gyrus more than lives up to the first part of that stereotype and utterly defies the second in his brilliant "Inside," an intriguing look at Faith's life behind bars.
Hth -- I have read precious little Willow/Tara slash, mostly because, as of this writing, the show has yet to really delve into their relationship in depth. Here's hoping they'll eventually be given the kind of complexity and beauty Hth accomplishes in her BTVS story "Twenty-Eight Days," one of the most poetic and ambiguous tales I've encountered.
Kipler -- Her take on the world of XF defeats the notion that shipper fic cannot be hard-edged, intelligent or insightful. Although all her stories at are fantastic reading, particular favorites include "Genius", a story that could easily have been an actual episode (and a great one) and "Strangers and the Strange Dead," a story of astonishing quality and subtlety that ranks as one of my all-time favorites in any fandom.
LJC -- Someday, maybe very soon, LJC's magnum opus in progress, "Eight Days," will see the light of day. Until then, though, enjoy the rest of her wonderful ATS fic, as well as the rich, dark-chocolatey goodness of the BTVS story "Glorious." Aspiring ATS writers would do well to consult LJC's terrific resources, including the invaluable Angelfic workshop list (?) and the Whedonverse Mary Sue Litmus Test (?).
MD1016 -- You should go on fanfic journeys with this woman, especially her really far-out, AU journeys such as "Journal 1999." However, you should not go on whale-watching journeys with this woman. She will back me up on this one. (Or, more likely, tell you not to go on whale-watching journeys with me.)
Rheanna -- What do I say about my beta, coauthor and friend? Only that her ATS fic amuses me ("For One Night Only"), intrigues me ("Blood and Water") challenges me ("In the Waiting") and sometimes moves me to tears (the incomparable "Vivere.") This is as good as ATS fic gets -- which is very, very good indeed. Read it all at her site, Dark City.
SpykeRaven -- If Angel/Lindsey slash is your direction, then "Tiger Lily" should be your destination. If it isn't, I can only tell you this story does what a lot of slash fails to do -- completely and convincingly bridges the gap between what we see on screen and what happens in the story. This is some of the best Lindsey characterization I've seen in ATS fic, and Darla is an unexpectedly strong presence throughout, even unseen.
Torch -- You read Torch's exquisitely beautiful TPM story "Crystallize" and have your entire definition of "love story" turned on its head. You then read her subtle, tricky West Wing story "One of These Things" and marvel at her ability to make the non-canonical completely believable. Then you learn that English is not Torch's first language, and you feel very incompetent and shlubby for about a month. After you get over it, you go back and read the stories again. And again. And again.
Zero -- When I was still a very new BTVS fan, I went searching for good fanfic. I was lucky enough to find Zero very early -- most particularly her brilliant "The Remembrances of Sun," which I still think is the most dead-on look at Spike's character I've ever seen. (No pun intended.)