Your point is well-taken, RM! What is a "Mafia" film? Though the GF Trilogy is my all-time favorite, I don't count it as a Mafia film. It's a film about a family, with the Mafia as the defining milieu. Analogy: "Gone With the Wind" is a film about a family, with the US Civil War as the defining milieu, but it's not a Civil War film per se. With that in mind, my favorite Mafia films:
--Goodfellas
--Little Caesar
--Untouchables
--Casino
--Scarface (original, with Paul Muni)
(I'd have rated several Mob-related films higher, such as Mean Streets and Bronx Tale, but they're not, strictly speaking, Mafia films.)