5. The Joker Returns
There's so much going on in The Dark Knight, from the incredible direction to Bale's performance to Hans Zimmer's booming score, that it's unfair to put all the praise on Heath Ledger's Joker, but his Clown Prince of Crime does steal the movie.
His exit is left deliberately open-ended too, and had Ledger lived it's a near-certainty that he would've returned to face off against Batman once again. As the character says in the movie: "I think you and I are destined to do this forever."
On a more purely craft level, though, it's likely Nolan would've wanted to work with Ledger again, because his performance is very un-Nolan like. The director is generally in complete control (he's not really an actor's director, as it were), and the actors he works with fit into that, rather than being method actors.
Ledger went the other way, pushing the Joker into a territory of complete and utter chaos, and it is the best performance we've ever seen in any of the director's movies, and the two reportedly developed a strong relationship. With the Joker still alive and Ledger not even close to what his peak could've been, you've got to think he'd have been back for more, which is something Ledger's sister, Kate, has said he believed too.