(Written in response to a reminder from Ryan Groff.)
These are not necessarily the best ten science fiction novels (in my opinion). They are the ten science fiction novels that affected me most significantly when I read them. And as a result, they are the ten such novels I care about most. I am grateful to their authors for the gift of their imagination, and for the ways in which each of these novels sparked my imagination, sometimes with profound personal consequences.
Orphans of the Sky, by Robert A. Heinlein
This may or may not have been the first science fiction novel I ever read. I read it in its Afrikaans translation, as Beheerkamer Onbeman (Control Room Unattended). And re-read it and re-read it. Undoubtedly this is the book that lured me into the world of science fiction. The translation was published in 1972, and I would have first read it sometime between the ages of 8 and 10.
The Word for World is Forest, by Ursula K. Le Guin
A novel of colonialization, I sometimes wonder if this is not the most important book I ever read - at least when considering its effect on me. The Word for World is Forest turned my moral imagination upside down, and converted me from my cradle racism. It did not convert me to a particular alternative, but it cut my affections from their tribal roots and left me in search of new moorings. It was first published as a separate book in 1976, when I was around ten, which is when I remember reading it. (My gratitude to the librarians and VERY well-stocked school and public libraries of my birth town, Bloemfontein, is unbounded ... even though those libraries were open to whites only during my childhood.)
The Dispossessed, by Ursula K. Le Guin
Turned me into an anarchist for all of my teens. Published in 1974, I'm pretty sure I first read it after The Word for World is Forest, but certainly before I turned twelve.
I love all of Ursula Le Guin's work, but none of her other books affected me quite as much as these two, or earned as much of my affection.
Dune, by Frank Herbert
Inclined my teens toward ecological concerns and Zen Buddhism ... and perhaps, upon reflection, the messianism that resonated with the Jesus I subsequently found in the Gospels?
A Canticle for Leibowitz, by Walter M. Miller, Jr.
Deepened my interest in the historical long term, the preservation of knowledge, and the cultural contribution of monastic communities. My second-favourite religiously themed science fiction novel of all time ...
... with The Sparrow and Children of God (counted as one book for the purposes of this list) remaining my all-time (harrowing!) favourite.
Neuromancer by William Gibson
Mostly because it introduced me to Gibson's writing, which I've grown to like more and more as each novel inched towards a "speculative fiction of the very recent past" that I resonated with most when I first read ...
Pattern Recognition by William Gibson
Also, Cayce Pollard will always be the best-dressed science fiction character ever.
The Diamond Age by Neal Stephenson
Partly, because it was a great escape-read during a year in which I could not escape the horror of my birth country's history (because I was working for South Africa's Truth and Reconciliation Commission at the time).
Counted as one, these novels of the future and the far past confirmed my Long Now-like concern for the long run (but hey: Isaiah 60). And nudged me closer to serious thinking about what is perennially human and what is historically malleable.