Ledger's performance will go down as iconic, but mainly because he died shortly thereafter. James Dean didn't have an ounce of talent but yet is still able to mentioned with Marlon Brando way too often. I think Ledger's performance is good, but I don't compare it to great performances.
He's essentially playing a monster role. Give him the credit for imagining the Joker that way, but that majority of the role is based on his interpretation of terror. Ledger abandons all of his personal qualities to pull off the role, but in doing so, he makes the role very open to easy mimicry from other actors. Nicholson didn't have the writing to back him up in the original Batman, but he did a perfect blending of his essential qualities as an actor with a fightful interpretation of the Joker. People complain that Nicholson is playing himself, but that's not true. They are just mad that they see any of his personality in the role, but that's what he had to do. I think actors have to transform roles to the strengths of their talent. Nicholson puts enough of himself in the role to make it an acting job. To study Ledger is to apply the right amount of make up, copy the physical gestures and get the voice just right. I thought a lot of Ledger's performance hinged on the right ingrediants that Ledger decided were scary, but he kept the role very simple. I can see more actors pulling off Ledger's interpretation than Nicholson's.