Double Indemnity, 1944
I think that Double Indemnity is the best example of film noir, if not my favorite. Fred MacMurray plays an insurance salesman who finds himself first falling for Barbara Stanwyck and then murdering her husband, only to be betrayed by her later on. It's the classic noir film, filled with tough-talking guys and rotten dames, and plenty of cigarette smoke and shadows.
What really stands out to me in this film is Barbara Stanwyck. I've read that she took on the role of Phyllis Dietrichson after Billy Wilder bet her to. I think that she really makes a kind of trite character into something really extraordinary. I think she's just fantastic in this role.