In time, when all the hysteria over TDK has died down, I hope Heath Ledger's performance in Brokeback Mountain will be hailed as his best. There is so much for an actor to do with a character like the Joker, so much opportunity to pull all the acting stops out. His performance as Ennis Del Mar was breathtaking and heartbreaking and it was all so subtle.
TDK was a good movie but not the year's best in my opinion. It'll probably get nominated, as will Ledger, who will probably win Best Supporting Actor but I can't see the Academy going for a comic book movie as Best Film. Then again, they did give 11 Oscars to Titanic so what do I know!
Brokeback is already acknowledged as his greatest performance and at least half of this posthumous Oscar buzz has to do with that role. If he wins it will be an award for his whole career, not just TDK.