Like everyone I have favorite stories and runs but it's hard for me to come up with a favorite single issue if a book. I'll say it has to be an issue and can't be Killing Joke or something like that. 1 single comic book. Like Captain America 227 or Detective Comics 156. Those are just random numbers, but you know what I mean.
Also, you can't make a list. In the spirit of 1 issue, you only get to pick one.