Those times were hard for Tony since he was having an actor's block (I assume this is a correct way of saying it) his personal life was not so good and keep messing with his profession. Then he met with WKW. Yes he is at the last few minutes of the film but he has filmed more than that (all ended up at the cutting floor). This performance opened his eyes into new things acting wise and he found a kindred soul in WKW as far as their art go. So this is one of the milestones in his career. He had such a hard time to give that performance and in the end it opened him up and improved him.

My favorite Tony performance - God, there are so many I don't know which to choose from. Of course he is excellent in WKW films and among them, Happy Together (although WKW is responsible from half of that performance) and In the Mood for Love.
Among non WKW films I have to go for Hard Boiled.