Can't pick one favorite. My head might explode.
LOL. I was just thinking I might have an orgasm thinking about them all at the same time. To help avoid either sort of metabolic disturbance, I shall simplify things by omitting neo-Noirs and non-English language Noirs.
No trouble in deciding my first two:
1) Out of the Past (Jacques Tourneur) 1947
2) Double Indemnity (Billy Wilder) 1944
As a (more or less) Londoner I have to squeeze in these two:
3) Night and the City (Jules Dassin) 1950 (A BritNoir/FrogNoir hybrid)
4) It Always Rains on Sunday (Robert Hamer) 1947 (BritNoir). Many BritNoirs contained a lot of social realism, as in this case.
5) Detour (Edgar G. Ulmer) 1945. Too down-market to be a truly great movie, but surely the purest essence of Noir in just 67 minutes.
Two I watch again and again when depressed. Bogart's dialog always cheers me up.
6) The Big Sleep (Howard Hawks) 1946
7) The Maltese Falcon (John Huston) 1941
The last "pure noir"?:
8) Kiss Me Deadly (Robert Aldrich) 1955
Two more "pure noirs" which help to define the canon:
9) The Asphalt Jungle (John Huston) 1950
10) The Big Heat (Fritz Lang) 1953
That is ten. You think I am finished, punk? I am a long way from finished! You will eat these and you will like it!
Angel Face (Otto Preminger) 1953. As revealed in another thread, I have a Jean Simmons fetish.
The Prowler (Joseph Losey) 1951.
The Verdict (Don Siegel) 1946
Clash By Night (Fritz Lang) 1952
Odd Man Out (Carol Reed) 1947 BritNoir
Nightmare Alley (Edmund Goulding) 1947 CarneyNoir
Murder, My Sweet (Edward Dmytryk) 1944. Another which defined the canon.
In a Lonely Place (Nicholas Ray) 1950
The Third Man (Carol Reed) 1949. BritNoir (though only one of the four leads was a Brit)
These are not central to the Noir canon, but probably greater as movies than most above:
Sunset Boulevard (Billy Wilder) 1950
Sweet Smell of Success (Alexander Mackendrick) 1957
Night of the Hunter (Charles Laughton) 1955
Can I have a few more picks? Another thirty should do it.