Sanjoy you can't ask this question of a bibliophile; it sends them into apoplexy! Pick one story? One favorite? How can anyone do that? That's so unnatural! As well ask to pick a favorite author! And space is such a huge theme with so many subsets! Aaaahhhh! I could spend days listing space stories and the merits of each one.
This photo shows just one wall of my living room. I have more bookshelves all over the house. You want me to pick one book? Fine. Read the Science Fiction Hall of Fame Series, (I guess that's four books, actually) some of the best stories ever written by some of the finest authors! I have them all in First Editions and no, I won't lend them out! Too precious by far.