Ledger, who died at 28 years old before the movie came out, completely transformed himself ... "I wondered how he was going to play the Joker, especially as Jack Nicholson’s take had been so iconic.

Joaquin Phoenix is officially stepping into the role of the Joker, and he's kind of terrified ... been portrayed by many actors, including Jack Nicholson, Heath Ledger and Jared Leto.

Batman fans will say Heath Ledger has the best live-action Joker, but that title should clearly go to Jack Nicholson in Tim ...