Double Indemnity is on just about every list out there. It's iconic, not the least because of Barbara Stanwyck's performance. I spotted a clip featuring her scene in the bath towel playing in the background on NCIS last night.
Fred MacMurray, probably best known for his Disney goofball roles in the Absent Minded Professor and the The Shaggy Dog, plays an insurance agent who winds up getting in over his head with an unhappy housewife who manipulates him into planning the murder of her husband.
Directed by Billy Wilder (who co-wrote the screenplay with Raymond Chandler, based on James Cain's book) the movie has some intricate, razor-sharp dialogue. There's plenty of action, too, but to get the most out of this movie, you have to really listen.