With all due respect to the late Heath Ledger, his con brio interpretation of the Joker was on a par with Cesar Romero's: More amusing than menacing, despite the mayhem and multiple murders. Excellent work, but not Oscar worthy.
My vote instead would go to the underrated Aaron Eckhart as Harvey Dent. Barely mentioned in the promos and dismissed with only a sentence or two in the reviews, Eckhart is superb in what amounts to three different roles: the "before" Harvey, the "after" Harvey, and [SPOILER ALERT!] the revenge-driven Two-Face.
Let's not let sentiment outweigh common sense. I felt the same way when NETWORK came out in 1976. Peter Finch had the showier part, but William Holden gave the better peformance: but Holden was alive and Finch was dead, so guess what happened?
The film was excellent entertainment but not an earth-shattering masterpiece. Chances are if Ledger was still alive other reviewers would feel the same and not let their hearts rule their heads.