1. Frankenstein, Mary Shelley. (1818) Read it online.
2. 20,000 Leagues Under the Sea, Jules Verne. (1870): It's Jules Verne. Do you know how hard it was to pick just one? Read it online.
3. Vril, Edward Bulwer-Lytton. (1871): Yes, I know, it's Mister It-Was-a-Dark-and-Stormy-Night himself. And Vril is not exactly the sort of story that will knock your socks off. But it's a must-read anyway; the grandfather of an entire class of science fiction. And who knows? You might like it. Read it online.
4. The War of the Worlds, H. G. Wells. (1898): What if aliens were as imperialist as we are? Read it online.
5. The Lost World, Arthur Conan Doyle. (1912): The King of all Lost World science fiction. Read it online.
6. Herland, Charlotte Perkins Gilman. (1915): A different sort of Lost World, this is the founding work of the feminist utopia genre. Read it online.
7. A Voyage to Arcturus, David Lindsay. (1920): Just think the words 'cult classic', and you'll start to understand just how odd this book is. C. S. Lewis described it in this way: "[S]cientifically it's nonsense, the style is appalling, and yet this ghastly vision comes through." In other words: a cult classic. Read it online.
8. Odd John, Olaf Stapledon. (1935): The superhuman, the utopian; something was bound to make it all go wrong. That something was normality. 'Angst' is a word that comes up a lot when people talk about this book.
9. Out of the Silent Planet, C. S. Lewis. (1938): A satire of scientism. A lot of science fiction aspires to be spiritual adventure as well; Lewis succeeds.
10. The Martian Chronicles, Ray Bradbury. (1950)
11. Foundation, Isaac Asimov. (1951): A little bit of cribbin' from the works of Edward Gibbon.
12. More Than Human, Theodore Sturgeon. (1953): Love, conscience, and outcasts united into a gestaltic organism by mental powers.
13. Childhood's End, Arthur C. Clarke. (1953): On the credit page are the words, "The opinions expressed in this book are not those of the author." It is a work saturated with irony; it has a message, but one never quite knows what it is. But that's not really surprising when you recognize how that fits with the story....
14. The Stars My Destination, Alfred Bester. (1957): Deep space is my dwelling place / And death's my destination. Very different from your standard fare. The Count of Monte Cristo with more cynicism.
15. Starship Troopers, Robert Heinlein. (1959): If you never read the book, but saw that Hideous Abomination of a movie, shame, shame, shame on you.
16. A Canticle for Leibowitz, Walter M. Miller, Jr. (1960): This book is absolutely amazing. It is one of the most exquisitely written science fiction novels you will ever read.
17. Dune, Frank Herbert. (1965): Herbert's writing has a lot of flaws. But who can resist his ability to make the extremely implausible sound plausible, and the successful portrayal of Byzantine-style plot and counterplot? And, hang it all, I want to see the Bene Gesserit's Book of Azhar.
18. Do Androids Dream of Electric Sheep? Philip K. Dick. (1968): Reality gets a little hard to pin down when anything - or anyone - could be fake.
19. The Left Hand of Darkness, Ursula K. LeGuin. (1969) What is gender? Also something very difficult to pin down.
20. Ender's Game, Orson Scott Card. (1986): You can read the short story on which it was based online.
Do you have any that you would add?