Come to think of it, has he ever been in a bad movie? The guy can make any part shine.
Look at "Under Suspicion"; practically the whole movie is him in a room with Gene Hackman, and it is better than a lot of action-based detective movies I've seen.
He brings a subtlety to his roles (and a lot of times a quiet dignity) that I don't think is matched by any other actor we have today. I also think he brings out a better performance from the actors he is co-starring with, he raises the bar and they have to bring their A game to keep up with him in a scene.
He is one of the few actors I would trust to see a film I had no prior knowledge of, just because he is in it.
He has only won one Oscar, and a few other awards, despite a huge list of nominations. He is under appreciated as an actor IMO.