Amazing Spider-Man #50
"Spider-Man No More!"
By Stan Lee and John Romita
Finding his life in tatters due to his secret identity as Spider-Man, Peter Parker decides to give up crimefighting for good; this only lasts until the end of the issue, but it leads to one of the best and most dramatic moments in the character's history.
The issue is famous for a number of reasons, including it's introduction of Wilson Fisk (The Kingpin) into comic book lore; it's a great one-off, and it kick starts the Kingpin arc that would continue through the next two issues.
The abandonment of the spider outfit proved to be so iconic that it was used by Sam Raimi in his sequel, Spider-Man 2 in 2004.