The first issue of Batman contained five stories, the brief two-page origin tale of Batman's genesis, and then the introductory story that presented the Joker for the first time. The other three stories featured Hugo Strange, "The Return of the Joker" and "The Cat," a preliminary version of the female criminal that became fully realized later as "The Catwoman."
This issue by itself established the primary Batman elements (and rules) that have earned DC Comics (and Warner Bros) some billions of dollars, and are still spawning the character as a force in other medias.
Bob Kane art from Detective Comics #32, 1939 issue, with Batman and handgun
Batman #45, Feb-March 1948, Win Mortimer Batman and Robin Christmas cover
Azzarello's Joker Graphic Novel