the worst thing ABOUT ledger's joker is that he detracts from all the other great Jokers of the past.
I honestly love all the Jokers (except maybe the new batman cartoon one, he seems a little... well... crazy, lol, but I haven't seen it yet, just pictures, so I'll hold judgement til then), and they're all worthy of praise for different things that they brought to the character.
Cesar Romero's Joker was a little zany, even by joker standards, but he was in a crazy medium, that played up the "Psychotic Clown" angle well. He was also what constituted the Joker in Popular culture for YEARS so you can't say his portrayal had no impact on future incarnations.
Jack Nicholson's Joker was the first "Dark" Joker that the mainstream got a look at, and he played the role admirably. He was a trickster, and a little childish, but he's the freaking JOKER. That's his MO. I also liked how Tim Burton actually explained the Joker, and connected him to Bruce Wayne, it made him feel a bit more important for the story, and properly gave him the place as nemesis. Too bad they killed him off in the first movie, cause if the Joker could have returned in the next batman movies, who knows where they could have gone.
Mark Hamil's Joker was for kids, but he delivered some of the craziest plots and lines of all time. You got to see more of Joker at his least comfortable, dealing with mindless idiots, and you got a real sense that he's not crazy, he just has a different perspective. That's an interesting point of view. Of course, Mark Hamil was merely the voice actor for the Joker, and other people were entirely responsible for his look and feel, but still, Mark Hamil's delivery was almost perfect everytime.
Finally, Heath Ledger's Joker.. Well, he's certainly the most "hardcore" of the pop culture Jokers. His make-up job was great, but I actually like the "acid twisted my face into a smile" backstory, rather then whatever happened to ledger's joker, which is never fully explained, and that's a shame. Heath's joker doesn't smile enough, he's not actually forced into that expression by the shape of his face, he merely looks like a kid who's put on too much of his mom's lipstick. he does manage to pull off the purple and green suit without making it look like a freak, which is an accomplishment.
His plots are always interesting and twisting, as they should be, and you will be laughing along with him if you're anything like me.
Heath Ledger played a great Joker, but please don't forget the shoulders that he was standing on, because they were all great in their own right.
edit: I know I didn't go into great detail with Heath's character and there's a lot more to be said, but this post was getting big enough and I wanted to end it.