Probably the most influential comic book ever published. Released in the June 1938 by National Allied Publications, it contained the first appearance of Superman.
The now-famous cover depicted the Man of Steel lifting a car over his head, and a 13-page story about the Big Red S written by Jerry Siegel and Joe Shuster led off the issue. Back-up stories featured cowboy Chuck Dawson, adventurer Tex Thomson, newsman Scoop Scanlon, and master magician Zatara. In addition to Shuster, the artists included Bernard Baily, Will Ely, and Fred Guardineer.
The Superman story was actually taken from strips rejected by Detective Comics. It featured a one-page origin story for the Man of Steel, along with Superman trying to save an innocent woman from execution while capturing the real murderer, coming to the aid of a woman being beaten by her husband, saving Lois Lane from a gangster, and investigating a corrupt Senator (by jumping around high buildings while carrying the politician to terrify a confession out of him).
Nowadays, a copy of Action Comics #1 in mint condition will cost you well over $3 million.