Who is the main protagonist in The Dark Knight?

The protagonist in The Dark Knight is surprisingly the Joker. He is still the villain but if you observe closely he has an agenda and Batman's only goal is to stop him. This would put the Joker as the film's protagonist and Batman as the antagonist. This similar style of a villain based protagonist can be seen in Bram Stoker's novel "Dracula" where the vampire count is the protagonist and "good-guy" vampire slayer Van Helsing is the antagonist, simply attempting to put an end to vampires deeds.