For me, one of the most memorable was Jason Todd.  After all, we killed him, really... It's not every day you get a chance to save a hero, but we turned into villains and finished him off.  And then there was the lasting effect it had on Batman for years afterwards...

Although it does make you wonder who at DC thought it was a good idea to bring him back.  After all, he was hated enough the first time that we wanted him dead!  Bet they wont give us that chance again.