I recommend "The Dispossessed" because it is a good example of how to write a philosophical novel. The story deals with the planet Urras, a modernist technological society, and the moon Annares, an anarchist Utopia populated by outcasts who fled Urras many generations ago. The plot in this book is really secondary. The purpose of "The Dispossessed" is to analyze both of these societies and to point out their strengths and their flaws. While I don't agree with everything that LeGuin says about community life on Annares, I do believe that she does a good job of displaying what life under such a system would be like. Despite being relatively short, this book covers a lot of ground, including religion, economy, sexuality, family life, and art.
With that said, I must confess a little bit of disappointment with the writing of "The Dispossessed". Having read the Earthsea Saga and "The Left Hand of Darkness", I know that LeGuin can do better. In particular, this book lacks any of the great descriptive passages found in her other works. A few good word pictures of the unforgiving landscape on Annares would have gone a long way towards making the book more intensely realistic. Some of the dialogue also falls a little bit short. Still, I view the book overall as being quite impressive, and a must-read for science fiction fans who like to think.