The "Joker Moment" that really sold me on Ledger's take is when he first meets with the mob bosses in (what looks like) the cafeteria kitchen. He comes in with a bit of a forced, stammering, "Joker laugh", he's wearing a fairly classic-looking Joker outfit (a purple suit), he does the "magic trick" with the mob guy's body gaurd, he sums up the state of the mob in the intervening time between the first and current movie, he predicts the next plot point (Lao's betrayal once caught), he presents his plan and then leaves. And along the way there's nice scenes of Joker's "insanity" like his "card" being a Joker from a deck of playing cards and then having the suit-coat filled with gernades tethered to his hand as an "escape plan."
The best part is when one of the mob guys asks if the Joker really thinks he can come in there, extort all of Gotham's various crime syndicates out of 50% of their massed earnings and then just be allowed to walk out the door. Joker says, simply, "Yes."
Other nice touches in there like when the Joker says to someone, pointedly, that he's non insane and other great moments. That whole scene was just The Joker, to me, all the way and sold Ledger's version of the character. Yeah, it's a bit disappointing it was more "The Joker's movie" than Batman's but Ledger's performance as the Joker was just so awesome that I didn't mind.
I just wish, again, that Eckhart's Two-Face could've been a similar grand treatment in the third movie because Two-Face is one of few others in Batman's rogues gallery who could carry a movie by himself.