Fantastic Four #2
The first appearance of the Skrulls. The story is as childish as the first issue, and I'm getting the sense that all of the Stan Lee-written issues are going be that way.
These early issues of Fantastic Four recall previous horror and sci-fi writers such as HG Lovecraft (more so in the first issue) and EC Comics's Weird Science from the '50s. The Skrulls are like something out of an EC Comics horror story, particularly the ending where they are forced to become cows with no recollection of who they were before.
It seems that the key of the early success of the Four is their team dynamic; that they are constantly squabbling with each other and are barely cohesive was probably a unique plot device in comics at the time. This makes me realize that comics still have a long way to go to reach the zenith of their literary potential, even in the 21st century. 1962 wasn't that long ago.