1. This is not a list of what I consider "essential" books. It is of books that I like. Some of them I consider downright inessential, but I love them. I have not put anything I consider "important" on the list if I just don't care for it very much.
2. One book per author.
3. Because I consider SF to be usually at its best in short fiction, I've allowed collections of linked short stories to stand as novels if I consider that the author's best.
4. Although all the authors are SF authors, I've occasionally used a specific book that I consider crosses the line into fantasy.
5. Many of these books I've re-read frequently. But some I've read only once; they just stuck with me.
Douglas Adams, The Hitch-Hiker's Guide to the Galaxy. In spirit, this seems to be more of a philosophical satire than an SF novel.
Poul Anderson, Three Hearts and Three Lions.
Eleanor Arnason, A Woman of the Iron People. The premier anthropological SF novel.
Isaac Asimov, The End of Eternity. I'm a little unhappy with this choice. Asimov's pre-1980s work is my favorite oeuvre of a Golden Age author, yet in this case it's conspicuous how much my regard is based on his short fiction.
Gregory Benford, Timescape. The premier SF novel about doing science.
Alfred Bester, The Stars My Destination.
Michael Bishop, Catacomb Years.
Arthur C. Clarke, Tales from the White Hart.
L. Sprague de Camp, Lest Darkness Fall. The seminal alternate history novel.
Philip K. Dick, Flow My Tears, the Policeman Said. Not the first PKD novel I read, but the one that really made me a fan of his work.
Thomas M. Disch, On Wings of Song.
Stephen Fry, Making History. Yes, that Stephen Fry. Has anybody else actually read this astonishing alternate history novel?
Lisa Goldstein, A Mask for the General.
Ken Grimwood, Replay.
Damon Knight, Why Do Birds?
Michael Kurland, The Last President.
Dave Langford, The Leaky Establishment. Really a bureaucratic satire, and barely SF.
Ursula K. Le Guin, Always Coming Home. I consider this fantasy rather than SF, though the author might disagree, but I also consider it the magnum opus, the single greatest work ever written by a genre SF author. Its place for me is like that of Dune for many other readers, and for similar reasons.
C.S. Lewis, That Hideous Strength.
George R.R. Martin, Tuf Voyaging.
Pat Murphy, The City, Not Long After.
Larry Niven and David Gerrold, The Flying Sorcerers.
George Orwell, 1984.
Frederik Pohl and C.M. Kornbluth, The Space Merchants. I'm unhappy with this choice for the same reason as for the Asimov.
Mike Resnick, Kirinyaga.
Kim Stanley Robinson, The Wild Shore.
Michaela Roessner, Vanishing Point.
John Scalzi, Agent to the Stars. This is the only novel by him I've actually read. I didn't get the impression I would like his later work nearly as much.
Robert Silverberg, Dying Inside. Fighting it out with The Book of Skulls, which might not be SF.
George R. Stewart, Earth Abides.
Amy Thomson, The Color of Distance.
Jack Vance, The Dying Earth.
John Varley, Millennium.
Kurt Vonnegut, Cat's Cradle.
Gene Wolfe, Free Live Free. Fantasy, I think, but the one perfect novel by an ambitious but often problematic - for me - author.
Roger Zelazny, Lord of Light. Fighting it out with Doorways in the Sand.
That's 1930s (1), 1940s (3), 1950s (6), 1960s (2), 1970s (5), 1980s (12), 1990s (6), 2000s (1).
From this list I think my weaknesses may be summed up as: 1) Humor; 2) Alternate history; 3) Local settings. There's a certain number of classic gutwrenchers here, but I think my preference is for well-wrought, intricate plots.
Authors whose short fiction I am particularly fond of, but who either wrote no novels or none that I think measure up, include: Fredric Brown, George Alec Effinger, David D. Levine, Robert Sheckley, William Tenn, James Tiptree Jr., Howard Waldrop.
Authors conspicuous for their absence altogether include: Lois McMaster Bujold, Orson Scott Card, Samuel R. Delany, Harlan Ellison, Robert A. Heinlein, Joanna Russ, Theodore Sturgeon, Connie Willis. A couple I actively dislike; the rest just don't normally do it for me, for one reason or another. This is not counting scads of authors of the last 2 or 3 decades that I've never seriously read.