His Girl Friday (1940)

I've seen this one numerous times and am convinced that it may be the quintessential Hollywood screen comedy, with its wry observations about love, unmatched breathless pace, and cynical, sharp satire on media and politics, all filtered through the uniquely American genre of the newspaper picture. It is also a triumph of the classical film style, under the masterful guidance of Howard Hawks, who never lets his technique show but instead makes it all look so deceptively simple.  One of those miraculous films where every element comes together flawlessly.