Re:Can Oscar's be given posthumously? (Score 5, Insightful)

Well, kinda...

Ledger's Joker is based more on the comic series which always had the Joker as more of a psychotic than a jokester. Perhaps it was the Adam West series's softening of the character that gave the Joker that impression in the general public. In the 70's when the Joker became even a bit more grittier, and through the Dark Knight and similar gn's, the "darkening" of the Batman universe came to influence Dini, etc. in how they developed the character for TAS, though a bit softer still since it was a kids show.

Ledger's version is easily the closest to the original Joker concept of a true psychotic criminal, one who not only revels in his own altered version of the world, but finds the humor in the differences of what is considered our normal and what he considers normal, and uses that humor as part of his villainy.
Nicholson's Joker, while following closely to The Killing Joke origin of the Joker (and in my mind a weak cause for the level of the Joker's psychosis as required for the original character), was more along the lines of the 50's and 60's prankster Joker.

Either way, it will take a lot to find another actor who can come close to bringing the Joker to life in the same way Ledger did.