Greedo said:
Once Upon A Time In America
and the Godfather 1 and 2 for me
+1
can't go wrong with those three tbh.
I also like John Woo's HK movies:
A Better Tomorrow 1 and 2
The Killer
Hard Boiled
Bullet In The Head
Ringo Lam's "City on Fire" is a classic too.
Any Japanese Yakuza films with Takeshi "Beat" Kitano (Violent Cop, Sonatine, Hana Bi etc)
The Original HK movie "Infernal Affairs" oozes class too (was remade by Scorsese as The Departed)