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The GF Trilogy is my all-time favorite film collection. But I don't count the films as "mob" movies per se. I think of the Trilogy as a family saga, with the Mafia as a defining milieu, but it's not strictly speaking a Mafia trilogy. To make an analogy: "Gone With the Wind" is a story about a family, with the Civil War as a defining milieu. But it's not a Civl War movie.
That said, "Goodfellas" is the best Mob movie.