I love C. J. Cherryh's science fiction. I've said it elsewhere (probably more than once) but no one does aliens like Cherryh.
Liz Williams has also written some excellent science fiction. I'd recommend Kate Elliot's Jaran. C. L. Moore is good for SF, but I wouldn't recommend her S&S. Ursula K. LeGuin has already been mentioned. Anne McCaffrey's early SF is good.
The Darkover books, as mentioned by Chris, are fun, but I'd call them science-fantasy rather than science-fiction, and they are very light-reading. Her early and middle period produced much better stories than her later books.
And for YA, I'm surprised that no one has mentioned Andre Norton.