What would it be?
Because let's face it, there have been some REALLY weird plot twists and retcons over the years. And while some of them are awesome (personally, I love Brubaker's Winter Soldier arc), some of them are... more than a little confusing. Like that time Magneto infiltrated the X-Men, or when Gwen Stacy was the parent of the Goblin's two children.
I would want to change The Killing Joke
I *loved* this comic when I first read it, and there are still some parts of it that I really admire. There's some great storytelling here. But in my opinion Barbara Gordon's shooting isn't one of them. I love what ended up happening (Oracle is by far one of my favorite heroes), but this part of the story just feels... careless. Even Alan Moore, the author of the book, feels that the decision was not a good one (Via):
"I asked DC if they had any problem with me crippling Barbara Gordon - who was Batgirl at the time - and if I remember, I spoke to Len Wein, who was our editor on the project...[He] said, 'Yeah, okay, cripple the bitch.' It was probably one of the areas where they should’ve reined me in, but they didn’t."
Gross. So, k'now, if even the writer isn't a fan of the story, maybe it did deserve a retcon.
Is there a plot line that you totally hated, a character you wish you could bring back (or alternately, of that would have stayed dead for a little bit longer), that you would go back and change if you were in charge of Marvel or DC tomorrow?