Dark Knight is one of those rare films were all the pieces (acting, editing, directing, music, writing, etc.) were strong, creating an unforgettable film.
Yes, Health Ledger was incredibly frightening as the Joker. As I sat in horror about what the Joker was about to do, he does something completely different which manages to be even worse. I think Ledger's Joker is one of, if not the, best villain of any story. He even out does Shakespeare's Iago.
But Ledger was not the only incredible actor in this film. Amazingly, all of the actors were strong in this film. Christian Bale not only portrays two sides of one man (Bruce Wayne/Batman), but he conveys all of Batman's emotions with half his face covered in a mask.
It is most unusual in an action film to have both great chases and violence and still not show quite everything - ones imagination providing an image and emotional shock that are impossible to forget.
An excellent movie, but those prone to nightmares after scary movies should see something else.