This is without a doubt one of the most impressive films I've seen in the last few years, up there with There Will Be Blood, in terms of commitment to the role, for Phoenix's feral Arthur Fleck is broken, beaten and scarred, both physically and emotionally. That is portrayed on screen to such an emotive level, swinging from utter dispair to psychopathy and back in the same scene.
The support all carry thier roles well, the scrip is tight and well written, and the score, while sometimes a little overwhelming, adds punch to key moments.
But its Joaquim Phoenix who just owns this film. While this could easily not be about Joker and still be amazing, he has given such an amazing take on the character, that I doubt future portrayals will stand up to scrutiny after this performance. Absolutely stunning film.