First appearances of Captain America, Iron Man, Thor, Hulk, Black Widow, Hawkeye, and The Avengers.
Natasha and Clint were originally supervillains before they reformed into heroes. Hulk was originally grey, but Marvel had to change his color to green because the grey kept bleeding through the paper. Tony was captured and held prisoner by Vietnamese commies originally.
there’s also some people called Ant-Man and the Wasp in there, but they’re not important