Ledger's performance was overrated, and he no doubt suffered from Dead Actor Syndrome, where everyone blows everything related to the person out of proportion.
Nicholson did a better job of bringing comic-book Joker to the big screen, and Mark Hammil is THE best Joker, period. Ledger comes at a distant third. His character may have been itneresting, but he suffered from the same problem as everything in Nolan's new, more realistic Batman movies (great though they may be): the Joker, along with Batman, Gotham City, and pretty much everything else, was never MEANT to be made realistic. The fun about the comics and the animated series is that they are dark, gothic fantasy. Once you turn Gotham from a gothic nightmare to AnyCity, USA, it instantly loses part of its character, and its charm. And the same goes for the characters.
The movies are good, mind you. I really, really do like them, but to me, they are more like an Elseworlds tale of an alternate reality in the DC universe, than a proper adaptation of Batman in the first place.