Your tastes sound very similar to mine. Diana Wynne Jones is one of my absolute favorite authors (though I loved Fire and Hemlock), and I also enjoy all the F/SF authors you mentioned–I can’t stand the epic quest BS either.
This is one of my favorite subjects. Don’t be alarmed.
YA fantasy authors:
Tanith Lee. Particularly her Unicorn series, and her newest Wolf Tower series. Excellent fantasy/SF books–she has an absolutely amazing imagination. There’s all these excellent surreal touches in her books. I love them. She’s well known for her adult ‘dark fantasy.’ I’ve never read it–not my cuppa–but her YA stuff is great, and not while darker than the usual stuff, not depressing or anything.
Patricia C. Wrede. Her Enchanted Forest series is YA-oriented, but great fun and not childish at all. I also recommend Mairelon the Magician and the sequel, Magician’s Ward. She also has some out of print fantasy for older readers–it’s more difficult to find, but it’s all pretty good stuff. She also co-wrote a wonderful book that has recently been reprinted–“Sorcery and Cecelia.” It’s a book about magic, set in Regency England, and excellent. And a sequel is finally being written! I’m very excited.
Megan Whalen Turner. The Thief and The Queen of Attolia, the sequel. The Thief won a Newbery Honor award, and it was Megan Whalen Turner’s first book. It was well deserving. It’s set in a pseudo-Greece setting, and the characters are complex and far above the level that many “adult” fantasy writers manage.
Neil Gaiman. Even if you don’t read his comic books, I recommend “American Gods” (admittedly not YA–and it’s not to everyone’s taste–but I adored it.) He’s also written a few other novels, including one or 2 that could be called YA, all of which I’ve enjoyed, though they are comparatively controversial. A lot of people don’t seem to think he does well as a novelist, but I disagree. Also, you’ve read Good Omens, right?
Kara Dalkey. Her duology of Little Sister and Nightingale, set in a mythical China, are definitely YA books, but not in a bad way. They’re free of the boring mundanity that makes some adult fantasy so boring, because they are about young people. That’s what I like best about YA fantasy. And, okay, I’m a sucker for romance.
Lloyd Alexander. His Prydain Chronicles are the most famous, but I like them least. They’re basically LotR lite, and while they’re excellent, his Westmark series is older, more mature, and gritty. It’s about the prices of war, and revolution, and it says some important things, but it’s all written in words a 12 year old could understand. And some of his other older work–particularly “The First Two Lives of Lukas Kasha,” “The Iron Ring,” The Arcadians," and “The Remarkable Journey of Prince Jen” are amazing fantasy books for YA and, I think, older readers. They incorporate the mythology and culture of non-English countries to an impressive degree–Arabia, India, Greece and China, respectively. They are written for kids, but if you can overlook that, you may enjoy them. Or heck, just give them to kids you know.
Hmm. If you don’t have a problem with homosexuality, I’d recommend “Swordspoint,” subtitled “a melodrama of manners,” by Ellen Kushner–if you can find it. It’s this absolutely gorgeous fantasy book about two men, one a hired killer, one a degenerate ex-student who is probably more. Needless to say, it’s very nontraditional (and not just because of the whole gay thing.) The dialogue is remarkable–funny and witty. There’s intrigue, and manipulation, and plots, and rebellions and it’s all razor-sharp and brilliant, like the edge of a sword.
I’ve been going on for way too long, and it’s late, but just one more–Lois McMaster Bujold. Read her and love her. “The Curse of Chalion,” at least, if not her space opera.