Crisis on Infinite Earths - Marv Wolfman & George Perez
I think I first heard of this while flipping through the book of Alex Ross' artwork. He painted the cover, which features more heroes and villains than you can shake a stick at: basically all the heroes from the DC Comics multiverse, including several versions of the Flash, a couple Batmans and Robins, two Supermans and a Superboy ... the list goes on and on. First published as a limited-run series in 1985, Crisis was meant to simplify the world of DC. When they resurrected Golden Age superheroes during the rebirth of superhero comics, they made subtle changes to their origins, and eventually resolved the inconsistencies with a parallel universe explanation. By the time Crisis was written, there were at least five Earths in the mix, all with their own histories and heroes. The promise Crisis made was: "Worlds will live. Worlds will die. And the universe will never be the same."
The idea was, by simplifying the DC universe and killing off lots of these variant superheroes, new readers could start with a clean slate, not needing to keep track of all the different parallel universes. However, as I read this book, I found that without knowing about all the backstory, it did get quite difficult to follow. The sheer number of brief appearances by lesser-known characters (sometimes simply to be killed off) was a bit overwhelming and seemed gratuitous. As well, the main villain behind the destruction of the various Earths simply refused to die, and it got a little tedious seeing the heroes repeat the same solution over and over.
I suppose it could be true that Crisis was a "defining moment in comics history," but I liked Kingdom Come better as a crossover comic.