Spider-man's "One More Day" killed the character for me. It's the only event where the very CORE of the character (great power, great responsibility) was butchered so painfully and miserably.
But, um, guys... Batman was JUST "killed" a year or so ago. Dick Grayson took over as Batman (and he was awesome at it), but Batman himself was catapulting through a thousand lives from the dawn of time or something to be reborn and reincarnated into a new form in modern day... or some crap like that. Seriously, Grant Morrison is like Alan Moore's level of crazy, only without the misogyny.
My personal worst comic book death was actually Azrael. Despite his low points in "Knightfall", Azrael was an awesome character who had an awesome series (for the first 50 issues), but it ended with him "dying", but no body was found. They teased his return for years and years... only for them to give him a throwaway death confirmation in Blackest Night.
Grr.....