Goodfellas probably has the best dialogue between characters than any film I've ever seen. Not just gangster flicks, but any movie.

Goodfellas
Scarface (not really a gangster flick though).

i've seen all the movies mentioned so far in this thread

and most of them are very good gangster movies

but one of my favorites is still Gangster Number 1

Casino

The Godfather, part I...part II is excellent, too. Part III...what the fuck was going on with that?

Of course, when The Sopranos, The Movie comes out...that will probably top them all.

Brutal
(Sicilian and VERY proud of it!)

Lock stock and two smoking barrels.
Snatch.

As for Hollywood big budget bullshit it would have to be.

Casino.

opps

1) Snatch
2) Lock Stock