I finally had time to see Batman 89, and after seeing the Dark Knight there is little doubt left in my mind. The winner of the definitive Joker contest is: Heath Ledger.
While I love Jack Nicholson, and I think he did a fantastic job in the role, and also stayed truer to the comic book in a few ways, such as using the Joker's trade mark weapons such as Joker Venom and Electric Joybuzzers, and preserving his warped sense of humor. His first appearance as the Joker was absolute classic, and for a while I thought this would be the definitive Joker.
Nicholson's preformance was still full of the campyness Ceaser Romaro brought to the character in the 1966 TV Show, and ultimatly clashed with the Gothic Atmosphere Tim Burton tried to establish with Batman 89, in many ways Nicholson tried too hard to imitate Ceaser Romaro's preformance, instead of trying to distance himself from it. The two scenes that really undo Nicholson's preformance are the Museum scene and the climactic parade scene, these two scenes ultimatly drag his Joker back to being little more then the goof ball of the sixties, and the use of music by Prince destroyed any sense of menace the character had when he first appeared.
In terms of Heath Ledger, there's little to say, where as the sense of pure evil you get when you first see Nicholson's Joker is destroyed through out the first film reducing him to little more then an ego driven clown, Ledger's Joker is about as close to pure evil as it gets. In fact, Ledger's Joker is so dark, so menacing that at times you start thinking that they actually went to Gotham City and recruted the Joker himself. What ultimatly sets Ledger and Nicholson's Joker's apart is that Ledger's portrayl preserves the Joker's dark intellect, and in many ways improves it, including convincing others to do his dirty work for him. The one feature that both Nicholson and Ledger's Jokers have in common is the Joker's passon for grand schemes.
While Ledger's preformance is about as close to perfect as it gets, it still had some flaws. The biggest of these is that many of the Joker's trademark weapons, such as Joker Venom, are not present. Another flaw that Ledger's Joker has is that he at times seems too invincible, for example in the scene where Batman is interrogating him, Batman smashes his head into a sheet of glass and he doesn't even get a scratch.
Overall, while Nicholson does have his marits, his preformance could have been a lot better, adding more of the dark tone that was becoming part of the comic book at the time, and seeing as how batman 89 was directed by master of the macabe Tim Burton, I was expecting a far darker tone to the Joker. Wheather it was studio pressure or some other reason, a definitive preformance was destroyed by including the campy attitude the Joker had in sixties. Ledger on the other hand, succeds where Nicholson failed partly due to the change of times, and the attempts to distance Batman from the sixties, and creates a Joker that has some of the Joker's humor, but is also a creature born out of your worst nightmare.