"Moscow on the Hudson" is his standout film for me. I was gobsmacked when I later learned that he doesn't actually speak Russian. His intonation and delivery were perfect in the scenes before he arrived in America. But even more than that there is the wonderful performance he puts in as a man who is completely overwhelmed in an alien culture. I like particularly his complete panic attack, almost like an aspie shutdown, when he goes to a supermarket to buy food, and is faced with too much choice... not just one kind of bread but twenty different brands. He goes to pieces in an utterly realistic way.
Has nobody else seen this film?
Also, I would say "Life According to Garp" ... unusually I preferred the film to the book, largely because Williams was so likeable, and so was Gene Close, playing the mother. In the book I found her very nasty, but Close humanised her.
And what about "Seize the Day?" The performance in that film should have won him an oscar... it's like Death of a Salesman, only even bleaker. Again, he acts a man breaking down with such utter conviction that I can't see how the actor didn't need psychiatric treatment just to recover from the performance.