I don't even hesitate to say Phoenix is the instant frontrunner for Best Actor at the Academy Awards, and I have zero doubt he will be nominated for his performance. My expectation is that he will win, because it's the best performance I've seen from an actor all year, and it's hard to imagine another performance with such nuanced intensity and power. The Joker becomes a living, breathing human manifestation of evil, and the film serves to both demystify him and also make it clear that even what we "see" of his origin is subject to question — unreliable narrators being what they are, the Joker being the most unreliable of all narrators, and the fact of his literal humanness doing nothing to remove our awareness something purely cruel and monstrous resides within his soul.
I've written a companion article to this review that you can read here, which goes deeper into the issue of the "sympathetic" portrayal of Joker and the controversy this has sparked by many film pundits and fans — most of whom haven't even seen the film yet, and most of whom also haven't even read the screenplay yet. Anyway, I'll avoid diving into that issue further here. But if you want to read detailed points about why the controversy is mostly a significant misreading of the film's themes, and why it's also bizarre in light of other film trends and critical reactions to certain other films, then go read my analysis of the issue here. Now, moving on…
There was a danger in Phillips' decision to take inspiration from Martin Scorsese, and in particular to aspire to a marriage between Taxi Driver, The King of Comedy, and Mean Streets — some of Scorsese's greatest and most dissected works. The result is frankly somewhat of a miracle, because succeeding at aspiration toward Scorsese's 1970s and early-1980s output is almost certainly doomed to disappoint… except Phillips succeeds.
Rather, Phillips doesn't just "succeed" — I will speak blasphemy and say he quite possibly succeeds in certain ways Scorsese probably wouldn't have.
Scorsese no doubt could've made this film, but it would've been even more grounded in realism and would've featured a more morally torturous personal hell through which the Joker navigated his way toward becoming what he is. Phillips achieves something slightly different, though, and in doing so captures a truer and more compelling essence of what the Joker really is — he builds a dreamlike realism, letting us enter the nightmarish perceptions and deceptions of the Joker, turning views grimy street-level authenticity into an altered state of consciousness. It all seems very real, and at the same time surreal, like a muffled, drug-induced experience of hyper-reality.