Yes, I will admit that I am biased. I grew up watching the Batman animated series starring Kevin Conroy and Mark Hamill.
However personally I have always believed that Mark Hamill (and yes I also know he was, and is Luke Skywalker) has always played the perfect blend of anarchist, jester and prankster in order to be batmans foil.
No offense meant to other Jokers as I have an enormous amount of respect for Ledger and of course Jack Nicholson's versions.
However, in the case of the legendary Jack Nicholson, I have always seen him as an actor who only plays variations of himself in his roles (I fully accept any contradictions as this is merely my humble opinion on the matter), and so have seen his portrayal of the Joker as merely an extension of himself.
In the case of Heath Ledger, I must of course begin by saying that I respect enormously his dedication to the role and yet I cannot but help think that his portrayal of the Joker is more suited to the role of the anarchy character of DC Comics.
I cannot wait to see Jared Leto's version of Harley Quinn's most famous beau, as well as seeing Conroy and Hamill duke it out as Batman and his most famous arch enemy one final time in the Killing Joke animated film.
While I am not saying that neither Ledger nor Nicholson have presented an entirely valid interpretation of the Joker, I believe that Hamill's version, whether it be in the animated series or the Arkham video game series provides a more balanced portrayal of the clown prince of crime, and I have to be honest as a comic book reader myself, whenever I hear the Joker in my head I always hear the voice of the man that I grew up with the insane cackle.