Odd to see a matchup from this movie but Thomas I suppose. Arthur was aggressive and vicious but physically he was thin to the point of being emaciated. In an actual fistfight I don't think he would win.
I'm curious to see how a joker like Arthur would actually function as a villain. He didn't seem very smart or malicious, only dangerously disturbed. I have a bad feeling about making a follow up movie with him as villain would be awkward or ruin the character. That being said, what a great and emotional roller-coaster of a movie.