Jack Nicholson plays a far more polarized Joker, doing horrible things and looking totally psychotic every moment he's on screen. And his quotable catchphrase blows Heath Ledger's out of the water.
If you removed Heath Ledger's Joker and replaced him with a different Joker or a different villain, this movie would be boring. Batman's boring, the crooks are boring, and Christian Bale does NOT know how to do Batman's voice