Hartmanis and Stearns, "On the computational complexity of algorithms", Transactions of the American Mathematical Society 117: 285–306 (1965)

This was the first paper that took the study of time complexity seriously, and surely was the primary impetus for Hartmanis and Stearns' joint Turing award. While their initial definitions are not quite what we use today, the paper remains extremely readable. You really get the feeling of how things were in the old "Wild West" frontier of the 60's.