The Dark Knight

The Dark Knight is a movie riven with bitter ironies and harsh truths, where ends start to justify means and the good end up corrupted. The glittering surfaces of the city mask the darkness lurking in the depths, which erupts into full view via the actions of the psychotic Joker. He claims to make no plans, to merely be the agent of chaos, and his irrationality makes him truly frightening. Yet the beautiful simplicity of the initial bank heist, where each of the gang kills one other member until there's noone left, verges on genius and he seems to have an unerring instinct for what will cause the most mayhem among the forces of good (a relative term in this film) Perhaps most impressive of all, not for one moment did I sit there thinking "that's Heath Ledger". The character grips the attention from the start and never lets go.
In one sense, the film is structured around a series of triangles that bring the moral conundrums into focus: Batman/Joker/Harvey Dent; Bruce/Rachel/Harvey Dent; Batman/Gordon/Harvey Dent; Bruce/Alfred/Lucius. Harvey Dent might be Bruce's rival for the affection of Rachel, but Bruce has hopes that the crusading DA might also allow him to put aside his alter ego. In fact, so many hopes are vested in Harvey Dent that at key moments the only thought of other characters is to save him. Batman goes to tremendous lengths to rescue him from the prison van and ends up sacrificing Rachel to save his rival/ally (although that was manipulated by The Joker), and Gordon's one thought during the hospital explosion is for Harvey Dent's safety, although by that stage it's too late. The Joker has intervened to "save" his enemy for the forces of evil. Where the Joker worships chaos, Two-Face will put his faith in chance as he hunts out those he blames for his fate including those who only ever wanted to save him.
The finale of the film undercuts the brief moment of optimism when decency triumphs on the two boats (and it's a lovely touch that it's only the hardened killer who has the common sense to throw the detonator overboard) only for the city's White Knight to fulfill his earlier prophecy. To maintain Harvey Dent's reputation in order to save the city, both Gordon and Batman must promulgate a lie. He died a hero and Batman is now little more than a loathed and hunted vigilante. The lies extend further of course. Alfred ensures that Bruce will never know Rachel finally chose Harvey Dent, leaving him his illusion of true love lost. It's a dark film where the sense of danger is palpable, although the violence is frequently off-screen or happens so quickly (the Joker demonstrating a point to the mob, using a pencil and a gangster's head) that the viewer can imagine the worst. The most horrific image it provides is Two-Face's burnt and ravaged features, a visual indicator of everything lurking elsewhere in the film.