Marvel x DC: World's Finest Marvel Team-Up

Marvel x DC: World's Finest Marvel Team-Up (usually shortened to just Marvel x DC) is an American comic book miniseries crossing over the universes of Marvel and DC Comics.

Relaunch
The series led to a relaunch of both Marvel and DC, referred to as All-New Marvel and  DC Forever.

Trivia

 * The comic is most likely the longest Marvel/DC crossover published.