My personal favorite, Jack Nicholson as The Joker in the first installment of the Batman franchise with Michael Keaton as Batman. Jack's ability as an actor made the role, arguably, the best portrayal of the character and perhaps most faithful to the original comic book character.
Caesar Romero as The Joker in the original Batman television series with Adam West as Batman and
Burt Ward as Robin. A caricature, a campy portrayal
by a gifted dramatic actor. He was part of a formidable
cast that included Burgess Meredith as The Penguin,
Frank Gorshin as The Riddler, and Julie Newmar and
Eartha Kitt as Catwoman.
Heather Ledger as The Joker. A diabolical and uncomfortable portrayal by a uniquely gifted actor, in a role that had a profound affect on the actor and had changed the actor's life permanently.
Joaquin Phoenix as The Joker. Another portrayal far different from the hammy performance of Jack Nicholson. Dark, violent and disturbing. What seems to be a common thread through the years is that the role seems to attract many gifted actors.