TheEnglishMaster:
I'm very ill-read in SF or SFF but I love Kurt Vonnegut's 'The Sirens of Titan' and 'Cat's Cradle'.
Also, I'd include 'Slaughterhouse-Five' (featuring its antiwar message) and 'Player Piano' (dealing with the effects of advanced automation) by Vonnegut.
A few others not yet mentioned in this thread (AFAIK) that I've enjoyed:
'Ender's Game' and 'Speaker for the Dead' by Orson Scott Card
'A Canticle for Liebowitz' by Walter Miller
'1984' by George Orwell
'Brave New World' by Aldous Huxley
'The Puppet Masters' and 'Stranger in a Strange Land' by Robert A. Heinlein
The 'Robot' novels ('The Caves of Steel,' 'The Naked Sun,' 'The Robots of Dawn' & 'Robots and Empire') and 'The Gods Themselves' by Isaac Asimov
'The Dispossessed' by Ursula K. LeGuin
'The Mixed Men' by A.E. van Vogt
'Skylark DuQuesne' by E.E. "Doc" Smith
I didn't include 'The Lord of the Rings' trilogy (by J.R.R. Tolkien) and two Fritz Leiber novels ('Conjure Wife' and 'Our Lady of Darkness') since IMHO they're basically fantasy rather than science fiction... --C.K.