Heath Ledger did an amazing job as a live action Joker but when it comes to Joker in the animated medium Mark Hamill forever.
Frankly I don't think you can compare a voice actor's work to a live action actor, both require different things when it comes to the performance.
-EDIT-
Both actors have the ability to give me chills, but when I read a batman comic with the joker in it; Mark Hamill is always cast for that roll. His laugh puts Ledger's to shame when it comes to sheer creepiness in a dark situation.
Frankly I don't think you can compare a voice actor's work to a live action actor, both require different things when it comes to the performance.
-EDIT-
Both actors have the ability to give me chills, but when I read a batman comic with the joker in it; Mark Hamill is always cast for that roll. His laugh puts Ledger's to shame when it comes to sheer creepiness in a dark situation.