Just so we are all clear, here are the tiers of gangster movies:
1. Goodfellas, Godfather 1, 2
2. Donnie Brasco, Casino
3. Scarface
4. Blow
5. Godfather 3
6. Carlito's Way 2: Rise to Power
7. Carlito's Way
The Departed is up there with Scarface IMO
Bronx Tale was a good ass gangster movie too. It wasn't as violent and it had a different perspective
There are some pretty good british gangster flicks like Snatch and Layer Cake
Lastly, City of God is a dope ass mob movie
As far as the OP question, Goodfellas.
The script,
slew of great characters, and acting