I borrowed a bunch of Marvel comics from a friend at school. There were a couple of the Pocket book collections of the early Spider-Man, Hulk, and Captain America.
But the first single issue I read that left an impression on me was Fantastic Four #250 by John Byrne. It was a fun story, but what really stood out to me was the guest stars Spider-Man and Captain America helped the FF fight a strangely antagonistic X-Men. The idea of a world full of heroes, each having their own adventures, meeting up and working together blew my mind. When I started buying my own comics, Marvel Team Up was my favorite title because it highlighted the aspect of the Marvel Universe that most appealed to me.