"Somewhere In The Night" (1946) directed by Joseph Mankiewicz. Sadly under-appreciated film noir--probably because John Hodiak's lead performance isn't very memorable. Steller supporting performances by Richard Conte and Lloyd Nolan.
The Blue Dahlia (1946), directed by George Marshall and written directly for the screen by Raymond Chandler. Chandler's novels were adapted for the screen and he adapted other writers' works, but this is the only time he wrote an original screenplay.
Director Jacques Tourneur's 1947 film noir masterpiece "Out Of The Past". Robert Mitchum, Jane Greer, and Kirk Douglas are all superb. (Greer then played her character's mother from this--brilliantly--in the 80s remake "Against All Odds")