Allow me to pimp this movie!
Le Samourai is cool, and will hopefully make the list as there are a few Melville fans on here.
One that doesn't seem to get watched as much, but is widely regarded as his masterpiece, is
Army of Shadows. It wasn't released in the US until 2006 following negative reaction from Cahiers du Cinema as to what they perceived to be right wing work promoting the then hated Charles De Gaulle. Melville didn't help himself, describing himself as a "right-wing anarchist", but those critics who slated him (after initially loving and being highly influenced by his style, especially Bob Le Flambeur) late reappraised the work, and in 2006, and now with Criterion and Studiocanal Blu-ray releases, there's no excuse not to see it.
The film focusses on a group of people that work as part of the French resistance against Nazi-occupied France. Melville himself entered the resistance, so its a highly personal piece of work, and probably his most human.
It's not easy to watch at times, it's dark and pessimistic, a real gritty examination of human life, of loyalty.
Anyway who's seen
Le Samourai or
Le Cercle Rouge, which seem to be the most popular around here, will know what to expect in terms of colour cinematography. This film is beautiful shot, just like those two, using blue/green colours to softly illuminate the dark world that the film inhabits.