Rocannon's World, 1966

I have become a big fan of Ursula K. LeGuin's work recently. Her prose is so tense and emotional, and the story backgrounds in her first novels (the so-called Hainish series) are pretty cool: most of the stories take place on colonized worlds far in the future, some of them savage but with some remnants of the old Earth civilization and technologies. So these societies usually are based on medieval or barbaric culture mixed with ultra-advanced technologies like faster-than-light drives, laserguns, antigrav gliders, etc.

It's hard to choose a favorite story, her first three novels Rocannon's World, Planet of Exile and City of Illusions are all similarly great. The Left Hand of Darkness (1969) and The Dispossessed (1974) both won the Hugo and Nebula award and are also bold pieces of fiction. Don't forget The Word for World is Forest (1975) which will be re-released soon, and which must have been a substantial influence for James Cameron's Avatar.

Le Guin's World - Hainish Encyclopedia