No one has EVER nailed the Joker, 100%, in live action. So far, the one to ever come the closest, was Jack Nicholson. His Joker is STILL the closest to being Comic Book accurate. Which is what I look for, most. Jared Leto was a close second.
I'm taking NOTHING away from Heath Ledger. His performance was amazing. My issue with his Joker, lies with the writers. They didn't do the character enough justice (no pun intended). I'd like to see a film that remembers that Batman Created the Joker.