hmmm, didn't we have a thread like this....maybe it was not limited to Science Fiction.
A few of my all time favorites are:
Frost and Fire - Bradbury
The Star - Arthur C. Clark
The Ones Who Walk Away from Omelas - LeGuin -
http://harelbarzilai.org/words/omelas.txt
Rescue Party - Arthur C. Clark:
http://www.baenebooks.com/chapters/0...498747___1.htm
The Story of Your Life - Ted Chiang
A Rose for Ecclesiastes - Roger Zelazny
Flowers for Algernon - Daniel Keyes.
A Boy and His Dog - Harlan Ellison
And there is this of course:
http://scifilists.sffjazz.com/lists_short_stories.html