The Lathe of Heaven was the first I read and had a big impression on me. Much later, The Ones Who Walk Away from Omelas did.

The WNET film of "The Lathe of Heaven" was wonderful. It was low-budget, and at times looked it, but captured the book well. It was unavailable for quite a while because of a scene centering around the Beatles' "With A Little Help from My Friends"; it was too expensive/complex to re-license it.