The Chaos Horizon
If I was only allowed, ever, to read one fanfic author -- no one else -- I'd name Kiki in a heartbeat. No thought required. The amazing touch for chacterization and the always dead-on-target dialogue, the gift for telling a story and the language to tell it well, the humor and the angst, the poetry and the one-liners.... nobody does it better. Stargate SG-1 (Eurydice Ascending, Somnambulist), Angel (The Powers That Be), Buffy (Myths and Shadow Plays, Disneyland), Due South (the incredible Exiles), Forever Knight (the hysterical My Evil Twin is a Vampire), Alias (the equally hysterical When Stavros Met Sophia), Firefly (Divergence), Smallville (Runaway Trains at 3 a.m.), Highlander (Let the Games Begin)... I could go on, but I imagine you get the idea. Read it all. Multiple times. God knows I have.
Elizabeth Ann Lewis
She's an undeniable romantic, and a damned talented one. Buffy AUs from dark to delightful (Fostering seriously rocks), Forever Knight and Highlander fic that'll make you remember you used to like the show, Babylon 5 for the hell of it, and a couple classic Firefly fics to round it out.
Dianne la Mercenaire's Vanity Fanfic Page
Dianne just doesn't write enough anymore, and it breaks my heart. < pout > Largely Buffy, and bouncing from the darkest to the silliest without missing a beat, but some solid Forever Knight and Vengeance Unlimited, plus the classic Due South slash series The Song Cycle -- the only slash I've ever read all the way through, and I liked it!
Celli and JenC's page, functioning not only as an archive for the pair, but as a good source of essays on writing. I'm in it mostly for Celli's stuff -- she's written a little bit of everything, gen to slash. Currently she's writing Alias, but there's Buffy, Highlander, Forever Knight... You get the idea. All of it's good.
BK the Irregular isn't the most polific of writers, but when he chooses to set words to paper (figuratively speaking), he always hits a home run. The Scarab is the finest Stargate/Buffy crossover in existence, and his deft touch also shows on several excellent Firefly stories. It must be good -- I don't make web sites for just anyone!
Missy has been writing in fandom long enough to everything you can do wrong -- and she doesn't do any of it. Her Duty, we get a short, snarky, hilarious look at how the Justice League really functions -- and the sacrifices that have to be made. For another giggle, check out her Farscape filk, "Brian Henson's Therapy", and everything she ever wrote for Gargoyles.
Julie Fortune: Fanfic for the Fearless!
Wow, this woman can write. Highlander, Pretender, Invisible Man, Stargate, Harry Potter... if the fandom has a dark side, Julie is irresistably drawn to it, and will suck you in along with her. Read it all, even if you're not in the fandom -- by the end of the story, you might be. Dark Avatar is a particularly well-done Slitherin-entric story, utterly true to Rickman's Snape.
Rheanna rocks my world, whatever she chooses to write. Possibly the best Basttlestar Galactica to date in Twelve. Some of the hands-down best Angel fic out there -- she captures the tone of the show, and the characters, perfectly, then takes it one step further. Vivere is one of the best future-fics I've seen, and For One Night Only takes a very old story cliche and makes it beautiful. Read it all. In Stargate, Totality makes the most of the Stargate universe in a universe-spanning series with some of the most engaging original characters you'll ever meet and a whopper of a villain. Roses in December is a heart-breaking Jack-centric story, and The Midas Syndrome is just neat!
Nomad can do it all. She deals in nicely-turned romance (the wonderful Jed/Abbey backstory And So It Goes), silly humor (As the White House Turns is a lovely laugh for the season 3 bitter folk in the house), serious drama (the Further to Fly series quite simply rocks the house down), and even the hugely improbable crossover (Donna the Vampire Slayer). There's Harry Potter Snape-centric fic I haven't read, but it's Nomad -- I'm going to assume it's good.
The Band Gazebo
I love Jeanine for her short, fluffy WAFFiness, which can take normal couples or, more often, completely UC couples, from almost any fandom and make them real and poignant. Her West Wing particularly rocks: Dark Horses is probably my favorite (a surprisingly lovely Sam/Carol series), but Novembers Past is good Toby/Ginger if you're in the mood, and Necklaces, Gifts and Names is just a kind of special WAFF. She also gives good Sweiss (Alias), plus Farscape, CSI and 12 other things.
Cagey is incapable of writing anything poorly. Seriously. A lovely touch for language, mostly displayed in drabble-equivalents, and she's willing to apply it to any fandom you care to name. I guarantee you will something you love here.
Beth has been known to write in several fandoms, including 'Profiler' fic; lots and lots of Sam/John goodness, both friendship and romantic. Fate is far too much fun and I love her series starting with Unorthodox Methods; I wish she hadn't gotten disgusted with the show at about the same time I did.