It seems fair that if I ask what the worst year is, that there should also be an opportunity for more positive discussion.
Best can be defined however you want it, although it generally means there was a unusually high number of good Spider-Man comics.
Some contenders...
1982- Roger Stern's Amazing Spider-Man had a fantastic batting average, with the Return of the Black Cat, Nothing Can Stop the Juggernaut, the Cobra/ Hyde two-parter and the Roxxon Tarantula saga. Mantlo's second run of Spectacular had the beginning of the Ock/ Owl war, as well as the debuts of Cloak and Dagger. And Dematteis wrote the bulk of Marvel Team-Up.
2001- Featured the beginning of JMS's Amazing Spider-Man, the debut of Tangled Web, the best of Jenkins's Peter Parker Spider-Man, and the finest arc of Ultimate Spider-Man (that showdown with the Kingpin.)
The spiderfan.org indexes can help you calculate what year a book came out. Keep in mind that the cover date is usually two months after the issue ships.