Well, some are easy, as they are generally regarded as classics, period. To name a few off the top of my head ...
Marvel Comics #1, first appearance.
Marvel Mystery Comics #8?, first superhero crossover, the battle with the Human Torch
Fantastic Four #4, Johnny Storm finds Namor
Fantastic Four #6, first Marvel super-villian team-up, with Namor and Doom
Avengers #4, Namor "frees" Cap from the ice.
Daredevil #7, the iconic mismatched, courage defining fight with Daredevil