Ahh, Mark Hamill. You didn't stand out that well as an actor in Star Wars, but your voice acting career is really impressive. He can go from calm ruthlessness, like his protrayal of Firelord Ozai in 'Avatar: the Last Airbender', to manic evil, like his iconic version of the Joker, originally from 'Batman: the Animated Series'.
Heck, in the latter's case, he is been so closely associated with the role, that when there is a version of the Joker that isn't voiced by him, people tend to be a bit confused, or flat out angry as was the case when they learnt that he wasn't voice the Joker in 'the Dark Knight'... this was prior to people knowing how well Heath Ledger was going to play the role, obviously.
Ledger did a stand up job as the character, bringing to life a character that merged together a bit from all the different interpretations of the character into one frightening whole.
But what if Hamill had actually voiced the Joker in 'TDK'? What would that have been like?
Well Pgirts over on the Youtubes has done a couple of videos in which he does an awesome imitation of Hamill's performance, but played from the monologues of Ledger's Joker. And I think that they're pretty awesome.
And as a bonus, here is another monologue with the same voice, but from Alan Moore's 'the Killing Joke',