One is not enough, and a lot of great movies where already named, still, some great movies are missing:

• Heat (Michael Mann) Every single time I see it, it is brilliant and I discover something new
• Jin Roh (The original animation movie), awesome atmosphere and only after the 2nd viewing one can really appreciate it
• Near Dark (1987) Why the hell did nobody ever produce something like this ever again?
• Miami Vice (Michael Mann), 'Style over substance', in a great way, although I have the shaky camera
• Seven Samurai
• Casablanca
• Strange Days
• Point Break
• XXX (Nobody understood that it was a parody back in the days :-P)
• What we do in the shadows
• Brazil
• Rocky
• Eternal Sunshine ...
• The city of lost children
• Leon the professional
• Dolls
• The Killer (The original of course)
• The last unicorn
• Dark City
• The thing
• The Lost Boys
• Spirited Away
• Donnie Darko
• Rashomon
• Brother (2000)
• Parasite
• Hatsukoi (First Love)

... from the top of my mind. :-P