If, after reading the books in the previous posts, and our imagined subject wished to continue, I would submit the following list of 10 more titles:
1) The Forever War by Joe Haldeman
2) Where Late the Sweet Birds Sang by Kate Wilhelm
3) The Hitchhiker’s Guide to the Galaxy by Douglas Adams
4) Blood Music by Greg Bear
5) Timescape by Gregory Benford
Gravity by Hal Clement Mission
7) The Stars My Destination by Alfred Bester
8) The Ringworld Engineers by Larry Niven
9) Again, Dangerous Visions edited by Harlan Ellison
10) Deathbird Stories by Harlan Ellison
Here we see Joe Haldeman answering Starship Troopers with his Vietnam War influenced The Forever War, probably the greatest science fiction war novel ever written.
In this list we also see the response in The Ringworld Engineers to errors discovered by fans in Ringworld. This illustrates the strong interaction between the fans and the writers in science fiction. In no other form of literature is this two-way relationship as well developed and entrenched as in science fiction.
Blood Music, Timescape, and Mission of Gravity may well be the three best examples of “hard SF” books in the genre, by 3 authors known for their hard SF.
The Stars My Destination could be considered as one of the ancestors of cyberpunk, and uses a favorite trope in science fiction, teleportation. It is considered by many to the one of the best science fiction novels ever written.
Where Late the Sweet Birds Sang is, like Le Guin’s book before it, a classic of post-apocalyptic science fiction regardless of who wrote it, male or female. Cloning is also a key element of this book. It, the Alfred Bester novel, and the Haldeman novel expand the tradition of literary greatness in SF novels to create works that should be appreciated beyond the scope of science fiction fandom.
Speaking of literary greatness, this is also the first time a collection of Ellison’s own stories appears. Widely considered to be the best short fiction writer in the field (if not the English language), it makes sense to include his work at this point. I could switch Deathbird Stories out for The Beast that Shouted Love at the Heart of the World. Both collections contain a number of Ellison’s most important stories.
The Hitchhiker’s Guide to the Galaxy is included because it is by far and away the most successful, the best known, and probably the best example of humor in science fiction. Humor is a big part of a genre that can sometimes seem to take itself way too seriously, and this book reflects that better than any other.
So far, this list of books represents, to me, not necessarily the best that science fiction has to offer (although it probably does include books that many would consider among the best in the genre), but the character of science fiction as a genre heavily influenced and even shaped by a relatively few writers.