Reading the first few issues of "The Fantastic Four." (Has nothing to do with the upcoming movie, just chance timing.)
They're not well written, at one level. But -- and this approaches a trivial observation -- in just this one book, the following characters appear in the first 25 issues (I went over to the marvel wikia for this, I haven't and am not going to read those 25) --
Reed Richards, Ben Grimm, Susan Storm, Johnny Storm -- FF#1, Nov 1961
Skrulls -- FF#2, Jan 1962
Ant-Man, Tales to Astonish #27, Jan 1962
Hulk - Hulk #1, May 1962
Victor von Doom -- FF#5, Jul 1962
Iron Man -- Tales of Suspense #39, March 1963
Watcher -- FF#13, Apr 1963
Nick Fury -- Sgt. Fury and His Howling Commandos #1, May 1963
Super-Skrull -- FF#18, Sep 1963
Captain America, Namor, and Thor all appear as well -- yeah, they weren't created during that period, but they had the good sense to throw them in there.
The Silver Surfer and Galactus appear in issue 48, glancing ahead.
Amazing outburst of creativity, that.