Let me suggest-

"Analyse This"
"Goodfellas"
"A Bronx Tale"
"Godfather Trilogy" (of course)
"Casino"
"Micky Blue Eyes"

That's just a few off the top of my head, that list should keep you busy the rest of the summer.

Let me suggest another movie "Heat" it's not a mafia movie but it stars Robert De'Niro and Al Pacino. De'Niro takes down scores and Al Pacino is a detective on his trail. Both as usual put on a powerful performance. A must see movie!!

"He who fights with monsters should look at it that he himself does not become a monster... when you gaze long into the abyss the abyss also gazes into you..."