Jack Nicholson, but an honorable mention definitely goes to Mark Hamill (and I'm not the first to mention it I see)
The reason I chose Nicholson was just because he played more what I always thought the Joker should be. Heath Ledger is great, easily the best part of Dark Knight, but I still prefer Jack Nicholson. He's more of a Joker, and I think Heath Ledger did a lot to make the other jokers of the past seem like.. well, a joke. But if you really like Batman, go back and watch the original Batman, and you'll see that Nicholson certainly has chops.
I liked the interconnectivity between Bruce Wayne and that Joker