Heath Ledger's Joker elevates The Dark Knight to the next level
Heath Ledger's take on the Joker makes The Dark Knight the best film in the trilogy. Ledger's Joker not only poses a physical threat to Batman, he is a mental force to be reckoned with as well. It is in this area that Ledger's Joker excels. Sure, the Joker is skilled with knives and gets the better of Batman countless times, going so far as blowing up the batmobile, but at the end of the day none of that matters if Gotham views Batman as a hero. Mentally though, Ledger's Joker is unstoppable. Throughout the film, Bruce Wayne's goal is to help Harvey Dent be Gotham's "white knight," the people's legitimate hero. Bruce Wayne understands that Batman, no matter how noble, will always be an illegitimate hero because he is a vigilante. Harvey Dent, as an honest elected official, offers the legitimacy that Batman lacks. The Joker knows this, he understands that physically beating Batman means nothing if Harvey Dent takes over. Ledger's Joker uses his mental prowess to outsmart Batman by corrupting Dent and turning all of Gotham against Batman.  Ledger's Joker is on screen for just 33 minutes, but his presence is felt throughout the film. His Oscar-winning performance drives the film without having the film focus on him. It's an incredible feat that demonstrates the excellence of the screenplay and acting in The Dark Knight. This is what elevates The Dark Knight above the other two films in the trilogy.
While Ledger's performance is great, it is not accurate to the source material. In the comics, the Joker isn't a physical threat to Batman, but Ledger's Joker is. Due to the fact that the antagonist of the movie is not a faithful representation of the character, this film is not the best in the trilogy.