What's your favourite older SF story?

[robkelk]

I have a reason to make a pseudo-representative list of older SF, so... What's your favourite older science-fiction story?

Two requirements:

• It must be at least 20 years old; and
  
• It must be set in the future, as of when it was written. (Macross' starting date of 1999 counts as "the future" since the show is from the mid-1980s, for example.)
  

Other than that, anything goes. Any medium (print, audio, visual), any language, any intended audience.

No need to defend your choice, and please don't denigrate anyone else's.

I'll start the list by offering my entry: Heinlein's The Moon Is a Harsh Mistress

(Edit: linked to the aforementioned reason)
--
Rob Kelk
"Governments have no right to question the loyalty of those who oppose
them. Adversaries remain citizens of the same state, common subjects of
the same sovereign, servants of the same law."

- Michael Ignatieff, addressing Stanford University in 2012