"Candy" (2006)
Certainly the most underrated performance of the later stage of Ledger's career, the actor returned to Australia for Neil Armfield's lyrical, if over-familiar, addiction drama. As Dan, the boyfriend of the titular Candy (Abbie Cornish), the actor looks like the Platonic ideal of the bohemian poet, who drags his artist girlfriend into the mire of heroin addiction, aided and abetted by their mentor (Geoffrey Rush, excellent as always). The two young stars have palpable chemistry together, and Ledger brings a charm and fierce intelligence that's a million miles away from Ennis Del Mar. But he's also typically free of vanity: Dan does, and instigates, terrible things in the name of love (love of heroin, that is), and the actor never sugarcoats them. It's a testimony to the alchemy of his performance that you only realize the extent of his toxicity to Candy as the film wraps up.

The Dark Knight

"The Dark Knight" (2008)
Taking on one of the most iconic screen villains was always going to be a challenge, given that Cesar Romero and Jack Nicholson had already delivered fairly definitive takes. No one expected Ledger to be the man chosen to don the make up, and certainly no one could have predicted his fierce, anarchic version of the character. And that's the key to his titanic performance: every tic, gesture, line-reading is unpredictable, making the character into a destructive force of nature that feels genuinely dangerous. The actor understands that the Joker shouldn't be funny to anyone but himself, and his skewed sense of humor is one of the most distinctive variations on the performance: despite attempts by the media to bring Ledger's commitment to the part into the narrative of his death, he told an interviewer while on the "I'm Not There" press tour that it was "the most fun I've had playing a character, hands down." And it shows. Even if Ledger had lived to see it, his Joker was always going to stand in the top tier of screen villains, and define the character for generations to come.