I'm pumping the brakes on the film's Oscar chances for now. We'll have a better idea in a month once the movie releases, but right now I'm getting a pretty clear vibe that this is going to be incredibly divisive with American critics. I don't think that's going to stop once the film's hit with 300+ reviews. It's already dropped from 87% to 77% on RT in just two days, and it's already borderline yellow on MC with just 21 reviews.
The Academy doesn't really go in for dark, nihilistic movies nowadays. The last one I can clearly recall is No Country for Old Men in 2007, but that film had glowing critical acclaim to back itself up. So here we have a dark, nihilistic comic book movie (let's be clear, not every Academy member is going to get over that), with middling reviews. This movie really needed the reviews to bolster its chances. I'm not so sure now.
In the end, if Joker gets a Best Picture nomination, I think Phoenix wins Actor. If Joker misses on Best Picture, Actor will go to Driver.