I've gotta go with The Godfather.
Why?
Because The Godfather is
UNIQUE.
Just about every movie dealing with organized crime and thugs that came
after it has to tip it's hat to Francis Ford Coppala and crew.
It was the
1st American film that gave us a glimpse into the
actual workings, the "ethics", the mindset and the brutality of organized crime or "The Mob" that we'd never seen before. Prior to it, gangsters weren't really fleshed out people. They were just "characters" more or less.
(Respect to James Cagney).
And IMO, neither The Untouchables nor New Jack City are really "gangster" movies.
Those are really
COP movies that show cops
fighting against gangsters and their crews.
Unlike The Godfather trilogy, Goodfellas or Casino which were actually
ABOUT those criminals/gangsters.
The fact that the gangster in those films (Deniro & Snipes) winded up getting the better juicier roles might make it easy for folks to forget that the movies aren't about them specifically.
It's like saying Avenger Infinity War or Endgame were about Thanos. When they were both actually about The Avengers
stopping Thanos.