Mockery and 'a humorously cruel tone' does not translate to joking; it translates to quips. Stylish quips, yes, and memorable ones, but still quips, not jokes. Don't get me wrong - I think Ledger made for a terrific Joker in terms of overall tone - he's certainly one of the more convincingly insane ones - but one of the central tenets of the Joker is that he JOKES. He sees himself as a clown, a comedian, a performer - also a criminal, an anarchist and a murderer, but his central persona is a comedic one. He glories in terrible puns with a deadly edge - he outfits himself with practical jokes gone wrong - he's his own best audience, who laughs at all his own jokes. Without that comedic edge, I don't think Ledger's Joker can really be termed the definitive one - he's just A Joker, with a decidedly different take. Now, if he'd wielded some cream pies filled with acid, or exploding cigars filled with nitro - THAT would have been a different story.