If we include Vonnegut
Cat's Cradle
A Scanner Darkly and Ubik and VALIS by PKD.
Neuromancer like everyone else.
Both Ellison's Dangerous Visions anthologies and also WHERE'S THE DV III, HARLAN? Plus Strange Wine and Ellison Wonderland.
Dhalgren by Samuel R. Delany.
The Lathe of Heaven by Ursula K. LeGuin.
The short stories of Henry Kuttner, which fucking nobody reads any more.
An odd book that also nobody reads called A Voyage to Arcturus by David Lindsay.
A Perfect Vacuum by Stanislaw Lem but this is barely even SF at all as it is just a bunch of purported reviews by literary critics of books that Lem made up, only some of which are SF at all.
Dying Inside by Robert Silverberg. Most of what he wrote was pretty flat, but this was amazing. It's about a telepath whose abilities have ruined his life, and now he's losing the one ability he ever had.
Most of the SF of the classic era that I even care about isn't novels at all, but the kind of short stories that got published in Galaxy and magazines like that when they existed, mostly before I was born. One example:
"A Pail of Air" by Fritz Leiber from the December 1951 issue of Galaxy. This is about a future where a black hole randomly crashes through the Solar System and drags off the Earth in its gravitational pull into empty space with no Sun, and the world that results, with the remnants of humanity trying to survive.
I haven't seen any reason to give a shit at all about any SF published in the last 20 years or so. It may say more about me than the quality of the work, though.