10. "Crisis on Infinite Earths" by Marv Wolfman, George Perez, Dick Giordano and Jerry Ordway (Crisis on Infinite Earths #1-12, plus a bunch of tie-ins) – 645 points (17 first place votes)
Crisis on Infinite Earths was both a love letter to the past of DC Universe while also the formation of a "new" DC Universe.
Marv Wolfman and George Perez put the DC Universe into a position where worlds were dying and realities were shattering. This allowed the pair to use a cast of literally thousands as they explored the vast realms of DC's comic history in a sprawling epic with more than one "Ultimate Battle Between Good and Evil."
The devices pushing this plot forward are the Monitor and the Anti-Monitor, one a benevolent being who was studying the DC Universe - the other a madman who wants to destroy the Multiverse, the backbone of DC's multiple Earths set-up (which allowed DC to separate their Golden Age creation from their Silver Age counterparts, but also allowed them to integrate comics they bought from other publishers without having to splice them together with their existing heroes).
In a battle this epic, deaths were bound to happen, and this story was SO big that two very big names saw their end - Superman's cousin, Supergirl...
and Barry Allen, the Silver Age Flash.
Initially, other titles were hesitant to tie into Crisis, but by the time the series ended, it was such a big hit that books were falling over themselves to tie into the event!
Wolfman and Perez ended the series with more than one magnificently diverse epic slugfests, until the dust settled and the DC Universe was never the same.
What a way to spend a Golden Anniversary!!