I might get shot for saying this, but I propose "The Dark Knight." It has all the elements of a crime movie: a terrific antagonist, a brooding protagonist, lots at stake, etc. Yes, there are more thought-provoking, art house films out there. But "The Dark Knight," in all its commercialized glory, will still be talked about for years to come. And that makes it the best crime movie of the decade.
Granted, some of them run together. The franchise has been rebooted about a million times. There was that terrible spat in the late 90s, with "Batman Forever" and "Batman & Robin." The latest incarnation is the best since the Tim Burton days ('89 and '92). Give "The Dark Knight" a shot. It is easily distinguished from the others because of its – for lack of a better term - supreme awesomeness. It's also a great crime movie, essential viewing for crime novelists.
I have a favorite, and not many people saw it. It's the quietly haunting film from Italy called I Am Not Afraid. I have never been able to forget this story of the kidnapping of a small boy and the relationship that develops with another young boy who discovers his whereabouts. Amazing movie.
The cool thing about this is the loyalty dynamic. The intense bonding between the captive child and the free child and the free child's relationship with his father. And the notion of the penalty that comes with doing the only truly right thing. The child actors had never acted before and the performances coached out of these children is nothing short of amazing. I have only seen it once but this movie still haunts me after 3-4 years. It is incredible.