Italo Calvino - Cosmicomics
I should really read this again, but I remember particularly liking The Form Of Space and The Distance of the Moon. There's an excellent comic touch to it all, as you might expect with Calvino. Rather wistful and I always thought of the stories as being somehow very Italian. There's definitely a strong influence from Calvino's interest in Italian folk tales, which works well with science fiction. Both are often concerned with one central idea.
Primo Levi - The Sixth Day and Other Tales
In Man’s Friend tapeworms compose beautiful poems to their human hosts. All the rest are great too, what with Levi being pretty much the greatest writer who ever lived. One's an excellent homage/pastiche of Calvino's style, particularly in Cosmicomics. As usual with Levi all the prose is stunning and it's actually worth reading Cosmicomics first just to see how incredibly he gets Calvino's style right.
Stanislaw Lem - Tales Of Pirx The Pilot and More Tales Of Pirx The Pilot
The dude wrote Solaris, if that doesn't make someone's sci-fi output essential I don't know what would. These are much funnier though, more like his excellent novella The Futurological Congress. Pirx is one of the great ordinary blokes of science fiction.
Garry Kilworth - Hogfoot Right And Bird Hands and Surfing Spanish Style
Both appear in In The Country Of Tattooed Men, which varies between science fiction and, er, generally odd short stories. Hogfoot Right And Bird Hands is a well executed tale describing the practice of forming pets from the amputated limbs of people so idle and machine-reliant they no longer need them. Surfing Spanish Style is a good dystopian Britain story about kids riding on top of trains due to having nothing else to do and no prospect of anything either. You can tell it was written under Thatcher, and it's all the better for it.