18. Crisis on Infinite Earths
by Marv Wolfman and George Perez
Published by DC Comics
This is the last of the big company crossovers on this list. I promise. And this one's here as much for its importance to the medium as for its amazing story that gave some (at least for a while) finality and maturity to the heroes of DC Comics. (After all, until the New 52 debut, DC's BC and AD was divided into pre-Crisis and post-Crisis.)
Sure, it's a bit of a downer with the death of Supergirl, Flash, and a cast of thousands of others, but it's also an amazing tale of sacrifice and heroism.