I think most of us will agree to Goodfellas and Donnie Brasco, which it's kind of obvious. Mean Streets also.

But to make an interesting discussion I will also mention The Godfather "Part II" as giving a pretty authentic portrayal of organized crime in the 1950s. Also, Lucky Luciano (1973), which is probably one of the most genuine portrayals of first generation Italian-American criminals, besides being an extremely boring film for the most part.