2002, Hong Kong, directed by Andrew Lau and Alan Mak
This one is a real find. While Hong Kong cop/robber flicks tend to be wall-to-wall action (not a bad thing if it's well done), Infernal Affairs quickly reveals itself to be a far more interesting creature as it explores the trials and tribulations of two moles, one an undercover cop in the Triads, the other a Triad who kept his nose clean and went through the police academy. If that sounds far-fetched, the directors do a skilful job of making it seem perfectly logical, helped enormously by the quality of the acting, from the stars to the supporting parts (it's a huge improvement on some similar HK flicks, with their inane comedy interludes). The directors have a great eye for detail, whether the planning of a drug trade or a police bust, and there's one fabulous scene that brings cops and crims together for a face-off in the police station. Since this is HK cinema, the bullets still fly now and then, but the plot is the real action here.