The ideal Joker? Actor Robert Knepper - he plays T-Bag in tv's Prison Break. I doubt there's an actor working today that is more suited to the part than Knepper. Two clips to show his amazing potential:
Can do creepy, looks the part - seems perfect. If he exaggerated his acting a little and did the mad laugh,he would be ideal. I can't think of anyone more suited than him. Doubt Warner Bros will find a better candidate than him.