Ubik by Philip K. Dick (it's hard to cherry pick his best work though, ask me a different day and I will choose The Three Stigmata or Do Androids Dream or something else)
The Stars My Destination by Alfred Bester (simply a good story and a great background)
1984 by George Orwell.
What all these books share is the idea that sci-fi is about social, cultural and economic dynamics, not ships or time travel.
That is my definition of sci-fi: An on-going essay about the evolution of the dynamics of society, using exotic technology/science merely as a catalyst.
Runner up: Ender's Game by Orson Scott Card
Could never finish (can't recommend): - Neuromancer - Cryptonomicon (Does the author feel entitled to mock the homosexuality of Alan Turing? Found that somewhat arrogant and in bad taste.)