Notes:

Most of the animated movies from DC are usually from previous comic storylines, just either streamlined for understandability or to make it easier to adapt to animation (with the exception of Batman: Year One and Batman: The Dark Knight Returns, which are almost word for word the same as the original comics). Most of them are great, but I've found out that The Joker's Last Laugh is not looked upon favorably by fans.

The only way it could be made into a movie and not suck would be to change most of the elements of the original story, and even then, it's doubtful the fans would give it a chance. I can't speak for those who actually run DC Comics themselves, but I bet those who are in charge of making the animated stories have an idea which comics would work and which wouldn't.