I think Ledger studied Nicholson's Jokr because there're slight hints of it sprinkled throughout his dialogue....
Like his laugh is almost like Nicholson's(
though I agree, Hamil's voice and laugh are iconicaly legendary) and the way he delivers some one-liners is done in an almost Nicholson-like tone.
Though personally, as for movie villains, Nicholson's Joker stands above the rest.
I'm hyped now....seems I'm gonna have to ask for a raise...'cause look to be cleanly broke next summer with all those movies.
