I would of said Civil War #2. Not only was it big put-on that you could tell was a big put-on, its soul purpose was to end the marriage and wasn't really about a story. They could of ended the marraige anyway, if they had a better way to do it, it would still be bad, but not nearly as bad. They promised the readers a big story and immediatly decided to just go one with their big event. The big 'event' was Spider-man. Not only can you attract new potential readers with a really daring story line, you don't have to make excuses about the books being bad because the marriage or the clone saga. Now they can't make excuses because it's a terrible book, and has nothing do with who presently is working on it.
On the other hand, you guys maybe right, OMD was even worse because its why CW2 was done (or not done). YOu can't really undo it (for us older readers anyway) because all we see is the ruination of the great character. Now he's a big idiot who will do anythign except think about what he's about to do and live up to the consequences.
You can tell the writers and artists really try on the new stuff, but it doesn't matter. 'Well gee, you guys did a story where everyone finds out who Spider-man is... then nothing happened. What do I care if Harry Osborn is alive again or there is some secret new goblin-like villian. I read an issue of it and everyone seemed like a washed-out version of their former selves. Parker and Osborn seemed to barely know each other. Ugh. I really thought Spider-man was the best character ever and man, did the destroy him. Who thought him being a back-stabbing weasel was a good plot line? Seriously. I don't think there's too much to debate that Civil War #2 and OMD ruined the character.