After Superman was killed at the hands of genetic superweapon Doomsday, superhero deaths weren't just for sidekicks and hotheaded Native American teammates anymore. Comic books had been playing stick ball in the sandlot for a while, but now their dad was going to sit them down and tell them hard truths about life. But then
Robot Superman showed up. And then Sunglasses Superman. And then Kid Superman, and Black Superman. And then Superman showed up again, too, because he'd been in a Kryptonian regeneration matrix in the Fortress of Solitude, because Superman has one of those, and because screw you, you idiot, why would you even think that DC Comics would make a single bold and irreversible move if they can cash in by going back on it? As if it wasn't a big enough "fuck you" that Superman came back at all, his brand new giant-ass MacGuyver mullet really hammered home how dumb the whole affair was going to be. Just as the death of Superman made death a reality for headlining superheroes, his revival made it clear that such deaths should never, ever be taken seriously ever again. And so it was, now and evermore.
Honorable Mention: Jason Todd/Bucky
Time was, comics fans knew a few things to be true: Superman is a dick, Batman has a contingency plan, and no one stays dead except Bucky, Jason Todd, and Uncle Ben. Then, in 2005, Bucky and Jason Todd both stopped being dead, tearing the fabric of reality asunder in the process. Jason Todd, AKA Robin, who had been beaten to death by the Joker, was revived when Superboy punched the wall of reality (please don't ask), and then further revived by a quick dip in a Lazarus Pit. Bucky turned out to have been frozen in a manner similar to his mentor Captain America, minus one arm, and was found and brainwashed by the Russians into being a Soviet black ops agent. What's most frustrating about these is that both characters' revivals were actually really, really well done, using Jason and Bucky to expose the darker aspects of Batman and Cap, respectively, and they've totally earned their continued presence in modern comics. This, however, doesn't change the fact that their returns totally undermine any fundamental certainty in comic books, nor does it make Uncle Ben any less lonely up there in comic book heaven.