Well that's an entire universe of topic; what do you define as Sci-Fi? Speculative fiction covers a lot of ground.
My favorite at the moment is probably
Do Androids Dream of Electric Sheep, by Philip Dick, one of the best science fiction writers ever.
I could also rattle off (in no particular order)
Dune,
Downbelow Station,
Starship Troopers,
Babel-17,
Diamond Age,
Neuromancer, and
Sundiver (and the rest of the Uplift novels) to cover off some mainstram works.
If you were leaning towards something a little different,
The Moon and the Sun is a terrific science fiction novel set in Louis XIV's court at Versailles. If you wanted to think politically,
The Dispossessed is a wonderful work about anarchy. Or
The Left Hand of Darkness, which deals with bigotry and stereotyping.
I could do this all day.