He portrayed that dark side of Joker forgotten by the animated series...
Ledger depicted The Joker as a very dark, real... and most important... CREDIBLE psychopath... unlike the foolish TV depiction. Personally, I hated the TV seriesīs Joker since I was a child (Mark Hamilīs Joker) - he seemed to me as a very foolish and silly villain... until I saw the Nolanīs films... then I loved him!
I continued to hate Hamilīs approach on the Joker (I think that he provided a wonderful voice... but the character was portrayed in a horrible way)... until I met ARKHAM ASSYLUM and ARKHAM CITY; those games really made a great use of Hamilīs voice putting it to a darker & sinister Joker!
However... "best" is a very subjective term... I think the right question should be... "Which one do you like/prefer?"