I think Walter Miller’s seminal work , “A Canticle for Liebowitz”, deserves mention. It is often overlooked.
I remember it as an excellent book, although I can't recall anything about it. I read it in my senior year in high school, which is some 28 years ago. Another terrific distopian book is Ira Levin's "This Perfect Day." Levin is best known for Rosemary's Baby. I believe that This Perfect Day has been out of print for some time now.
Mark