BlueSunCorp (how can you not love a site named Blue Sun Corp?) has a list of their top 10 science fiction novels based on ideas, strong characterisation and good writing -all 3 elements must be present.
I like the way these people think!
Here's their list with the ones I've read bolded:
Dune, by Frank Herbert
The Left Hand Of Darkness, by Ursula K. Le Guin
Helliconia Spring, by Brian Aldiss
Tschai, by Jack Vance
Neuromancer, by William Gibson
Red Mars, by Kim Stanley Robinson
The Man In The High Castle, by Philip K. Dick
The Book of The New Sun, by Gene Wolfe
Pavane, by Keith Roberts
The Forever War, by Joe Haldeman
The site has detailed descriptions of each book, describing how each meets the criteria set out.
I haven't read Dick's The Man In The High Castle because I do not like alternative history novels at all.
I have never even heard of the Tschai series, though I've read several books by Jack Vance. I'm putting that one on my list and will pick it up soon.