Justice League of America #21 (1963)

In this early adventure of the Justice League of America, readers are treated to a true gathering of champions with the first ever team-up between the JLA and the Justice Society of America.  When villians from both Earth-1, the mainstream Earth of DC continuity, and Earth-2, the alternate reality where the Golden Age heroes of the DC universe remained for years, combine forces and commit crimes across both universes, it is only the combined power of the premiere super teams of two universes that is capable of preserving justice and order.  This classic cover depicts the Justice League trapped in their headquarters and calling out across the dimensions to the Justice Society for assistance.  Penciler Mike Sekowsky, inker Murphy Anderson, and letterer Ira Schnapp bring together this seance-inspired, historic teaming of legends.  Notice especially the Martian Manhunter of the 1960s, which more resembles Lex Luthor with a digestive disorder than the Martian that fans of the Modern Era know so well.  

Shazam! #1 (1973)

Originally published by Fawcett Comics from 1939-1953, Captain Marvel (affectionately known to his enemies and fans alike as the "big red cheese") was acquired as a licensed intellectual propery by DC Comics in 1972.  Interestingly, it was due to a copyright infringement lawsuit, claiming that Captain Marvel was a blatant and illegal infringement of the Superman character, that contributed to the end of the original run of Fawcett's premiere hero.  The first issue of this revival introduced the Fawcett universe as having been in suspended animation for 20 years before finally breaking free.  This explaination, already hackneyed and laughable in the early 1970s, was eventually edited out of continuity as the characters of Captain Marvel, the wizard Shazam, the Marvel family, and their rogues gallery progressed from an alternate universe entirely, to part of DC's multiverse, and then finally to a wholly owned franchise of DC Comics and inclusion within its complex paradigm.  In retrospect, this premiere issue (complete with a tacit endorsement from an ever-smug Superman) begins the Captain's long path back to respectability and relevance with art by C.C. Beck, Nick Cardy, and Murphy Anderson.