"Ubik" by PKD
Brilliant story, read it a while back and found it in a 2'nd hand shop recently -decided I had to own it
The Culture novels by Iain M Banks (start with Consider Phlebas)
' Culture-novels, I'd recommend beginning with something where The Culture
itself is a little closer, The Player of Games
And, of course, his non-Culture SciFi is brilliant space-opera -even if he limits himself by not having FTL flight
Rendevous With Rama and the Odyssey series by Arthur C Clarke (well, anything by ACC, actually).
The Foundation and Robot series by Isaac Asimov.
Friday by Robert Heinlein, as well as just about anything else he wrote.
Of course... even if I do
find most of Clarke a quite tedious read.
If you can find it: "Terra" by Stefano Benni
is both fantastic, funny and a thrill-ride of strange environments and people.
Aldous Huxley's "Island"
is a down-to-Earth utopia where the science is pretty real (for a 1962 novel).
Philip K. Dick is also worth a read. Mostly because his novels are a better read than the films Hollywood made loads of them into.