My personal favourites would have to be:
Joan D. Vinge
Truly excellent and one of my personal favourites. Best is the Tiamat Cycle, though the Cat books are also good.
Maureen F. McHugh
Also wonderful, often with a low-key, poetic subtlety. Best bets are
China Mountain Zhang (a minor classic) and her short story collection
Mothers and Other Monsters,
free as a Creative Commons download from the publisher (I own a paper copy)
Lois McMaster Bujold
Marvelously characterized characters who have to both think and fight their way out of sticky situations using everything they've got rather than simply go out guns-ablazing.
Diane Duane
Puts even a scientific basis into the explanations for her fantasy works. Also one of the rare few writers who can write an actually good and believable
Star Trek novel.
Nancy Kress
I'm a sucker for societal consequences of genetic engineering stories, and Nancy Kress obligingly provides about 45% of my fix*. Excellent stuff, but she reads much better at short story and novella length than in her novels, I find.
Best bets:
Beaker's Dozen and
Trinity and Other Stories short story collections though she should have some good stuff in the other collections I haven't read. Also,
Beggars in Spain novel (sequels are okay, but tend to overthink things in a weird direction) and if you like sf thrillers,
Oaths and Miracles and
Stinger.
Connie Willis
A little more hit-and-miss than Nancy Kress, I find, but when she's good, she's very good and when she's not so good, she's still moderately entertaining. Seems to work best at the two extremes of either fairly long single novel, or fairly short story. Best bets:
The Doomsday Book and
To Say Nothing of the Dog at novel length (two-part sequel
Blackout +
All Clear was kind of a letdown), and her short stories are often very funny and/or poignant and available in 4 collections, 2 of which are still in print and come with author's notes on each story (
Fire Watch and
Impossible Things, available as e-books).
Jo Walton
Has written comparatively little, but what I've read has been of a very high calibre. Only really sfnal long work of hers is the Small Change trilogy, set in an AU where Britain became fascist after avoiding WWII. It is, of course, excellent and recommended.
I've also enjoyed works by:
Suzette Haden Elgin
Mostly writes language self-defense books these days, but back in the day, her
Native Tongue trilogy was a creepily-
Handmaid's Tale-like exploration of the potentials of alien communication and the status that would give linguists in a world where women were oppressed chattel and had to work to devise an especially secret means of communication to freely express themselves.
Octavia Butler
I've only read short stories of hers when they showed up in those
Year's Best Science Fiction volumes, but what I read, I liked.
Judith Tarr
Mostly writes very good historical fiction/fantasy, but did some wickedly funny alternate history short stories. Go read her two stories set with
Elvis as President of the US and the Kennedy brothers as a rock band and the
cult that arises around them both, over at the BookViewCafé for proof.
Tanya Huff
One of my favourite authors for fantasy. Her sf short stories never really grabbed me much, though, but I'm enjoying her Valor MilSF series.
Marion Zimmer Bradley
A lot of her sf work is kind of dated, and she's better known for her fantasy franchises, and there was that thing where she was accused of covering up for her pedophile second husband, but she did write some very good sf books in the middle of her
Darkover cycle which are still well worth the read.
Best bets:
The Heritage of Hastur and
Sharra's Exile, which really form the core of the series.
Stormqueen!,
The Shattered Chain and
Thendara House, and
The Forbidden Circle are also quite good. And I have a special fondness for
Hawkmistress!, although some people have called it a Mary Sue wish-fulfillment kind of book as far as the main character goes.
The
Marion Zimmer Bradley's Darkover short story collection probably gives the best overview of the series and is available at a low cost DRM-free (even lower in conjunction with discount coupon, like the 50% off one Fictionwise had over the weekend which was still working when I checked it this morning).
Anne McCaffrey
Although her writing has seriously deteriorated over time and was mostly only middling-good to begin with, I have a certain liking for the Pern and Ship Who Sang series, which did have some very good stories to start with. Best bets:
Moreta, Dragonlady of Pern,
The Ship Who Sang, the first few "adult" Pern books (
Dragonflight/
-quest/
White Dragon, also think
Dragonsdawn is pretty good) + the YA Harper Hall trilogy (
Dragonsinger,
Dragonsong,
Dragondrums if you feel like it).
Cherie Priest
I liked the two steampunk novels of hers that I read. Have yet to try anything else.
Well, this is everyone I can think of off the top of my head. I'm sure more will spring to mind later.
* Brian Stableford provides the other 45%, plus a few other authors for the final 10%.