$\begingroup$

This question is (inspired by)/(shamefully stolen from) a similar question at MathOverflow, but I expect the answers here will be quite different.

We all have favorite papers in our own respective areas of theory. Every once in a while, one finds a paper so astounding (e.g., important, compelling, deceptively simple, etc.) that one wants to share it with everyone. So list these papers here! They don't have to be from theoretical computer science -- anything that you think might appeal to the community is a fine answer.

You can give as many answers as you want; please put one paper per answer! Also, notice this is community wiki, so vote on everything you like!

(Note there has been a previous question about papers in recursion-theoretic complexity but that is quite specialized.)