I would suggest Arthur C Clarkes The Sentinel but I've been beaten to that one
It is probably the best example of what a good short story can carry with care and feeding.
Note also that it was written as a BBC competition entry but failed to place. So don't despair, all you writing challenge entrants.
It was the basis for 2001 A Space Odyssey.
Id love to a faithful adaptation of The Sentinel as Clarke original wrote it .
