#8 The Foundation Trilogy by Isaac Asimov

Asimov didn't write Science Fiction here, he wrote a book about politics and sociology set in a futuristic world.  It is not about space travel and wondrous technological gadgets, it is bout how people use these things to shape their world, deal with problems and save the galaxy form going back into the dark ages. He has made an allegory set in space to the real fall of an empire in Rome.  In doing so he writes an essential piece of literature that absolutely everyone should read.