That is from a reader request, please note I am not saying these are the best (that would be a separate query). Here goes, noting I am engaging in some bundling of volumes and sequels:
1. Olaf Stapledon, Last and First Men, Star Maker. Who needs characters and plot when such a compelling mega-Hegelian take is on the table? His other novels are underrated as well.
2. Isaac Asimov, original Foundation Trilogy. But no, the books didn’t want to make me become an economist and in fact when I read them at age fourteen (?) I recoiled at their historicist, anti-Hayekian, and anti-Popperian nature. I, Robot is actually a more important book, and one of the most influential of its century, but it is less fun to read.
3. Stanislaw Lem, Solaris, doubles and erotic guilt, with a touch of Girard, check out the Tarkovsky film as well.
4. Ursula LeGuin, The Left Hand of Darkness, her masterpiece, sadly I find The Dispossessed pretentious and unreadable.
5. Arthur C. Clarke, Childhood’s End. In for a penny, in for a pound, as they say. And once again, why haven’t they turned this into a movie?
7. Larry Niven, Ringworld. Read this one through the lens of Dante.
8. China Mieville, Embassytown. It demands serious attention, but worth a try even if you don’t enjoy his other books.
9. Liu Cixin, The Three-Body Problem trilogy. Again note the first volume is tough sledding for quite a while.
10. Orson Scott Card, Ender’s Game trilogy, it only gets great at the end of the first volume, nonetheless deeply worth it.
Assorted notes: I would have said Dune, except that last year I tried to reread it. John Wyndham deserves a lifetime achievement award. Philip K. Dick is “idea rich,” but basically a bad and overrated writer. And don’t kid yourself, Neuromancer, while important, isn’t that much fun either. A big chunk of Verne and H.G. Wells is worth reading, more than just the famous ones. I’m a fan of Neal Stephenson, but not sure my favorite works of his count toward this category. Huxley’s Brave New World would make the list if it counts. Gene Wolfe is OK, but no need to lecture me about him in the comments, same for Ray Bradbury. Some Heinlein holds up fine, but most does not. Vonnegut no, but I like Neil Gaiman’s Sandman series if it counts as science fiction. There is also Iain Banks.
Honorable mentions: Joe Haldeman, The Forever War; Greg Bear, Eon; Octavia Butler Xenogenesis trilogy; Mary Shelley’s Frankenstein. My dark horse pick might be Michel Faber’s The Book of Strange New Things, or Audrey Niffennegger’s The Time Traveller’s Wife, if that one counts as belonging to the genre. High marks to Stephen King’s Dark Tower series, and The Stand, again if they count. Any of these mentions could make the top ten without shame.