Phoenix did a phenomenal job in Joker... however, the movie was a mess. The first two acts crawled painfully slow and the third act was not strong enough to carry the film home. You can not go from a position of severe mental illness to an 'every man'... there is too much of a disconnect. If the first two acts were shortened to illustrate a "one bad day" slip into madness then the film would have been much more cohesive. The movie took great pains to illustrate Arthur's life long mental illness... which makes for a great political statement... though it's not what the Joker is about.
The tragedy of the Joker is in that anyone can slip into madness. By starting with mental illness it's a not so much a story of how or why just when... and that's not as compelling or entertaining to watch.