I don't like to title a list "best" because it's such a subjective word, so I called this my favorite books. I have been reading science fiction for decades and these are the novels I remember most fondly. There are some seminal works that aren't on here because I didn't like them (like CHILDHOOD'S END, which I agree is a good book, but I didn't enjoy the ending).
If Jane Austen wrote science fiction, she could have written this book. It's a novel of manners, romance, and adventure, in a far-future setting. I've read and enjoyed almost all Bujold's books, and I still like this one best.
This is a real classic that portrays what kind of culture might evolve if you turned the moon into a giant prison colony. I think it stands the test of time a lot better than some of Heinlein's other works.
I am having trouble editing this list! This is supposed to be "To Say Nothing of the Dog" by Connie Willis. She writes beautifully crafted books that are often funny and sometimes very sad. This time travel novel is funnier than most and not at all sad. I can't make myself read her DOOMSDAY BOOK based on what I know of the ending.
I can't say what I loved about the story without giving away a crucial plot point. Vinge does a fantastic job of setting up a unique species and then letting the reader figure out for himself/herself how they're unique.
This is my favorite of all the Heinlein "juveniles," most of which I consider to be better written than his adult novels. For one thing, the characters have real families and real relationships. Ad of course, the space-faring traders have a cool culture, too.
LeGuin is on my list twice because she writes about fundamental issues. It was gender in THE LEFT HAND OF DARKNESS and it's about fairness and human nature in the this one. It's grimmer than my other favorites but still earns a place.