I agree with so many that have been mentioned. One more vote for "The Lives of Others". A brilliant movie!
As far as acting is concerned. No one can top Elizabeth Taylor's transformation into a pathetic, codependent, alcoholic in "Who's Afraid of Virginia Woolf" She won the Oscar for it. And she deserved two for that performance.
And since we are on the subject. "Cat On a Hot Tin Roof". With Paul Newman, Elizabeth Taylor, and Burl Ives should get a mention.
Also, going way back.1933. "Dinner at Eight" with all three Barrymores, Jean Harlow, Billie Burke, among many others, makes my list.