Thanks to those Pocket Book reprints from the '70s and '80s I got exposed to a lot of the early stuff. The first few times the Fantastic Four met up with Doctor Doom was great, culminating in Doom launching the Baxter Building into outer space; also Galactus. Spiderman's run ins with Doc Ock were very arcy. The Green Goblin arc, ending in the Death of Gwen Stacey is a classic. The Secret Wars are some of the first comics I remember reading, and taught me far more than the cartoon and live action series just why one should never mess with Spiderman. A Lonely Place of Dying was one of my favorite Batman arcs, along with Tim Drake's early years as Robin- barring Dick Grayson, who was Nightwing through most of my life, Drake is the best Robin. Batman: Year One, The Dark Knight Returns, Watchmen. X-Men: Days of Future Past. The first few arcs in Superman/Batman comics were great, did an excellent job of breaking down the last of the post-Crisis "Batman and Superman work with each other, but don't like each other" attitude, and the one offs were hilarious.
And of course, all of Atomic Robo.