Scorsese's best work is Taxi Driver and Raging Bull IMHO, those are films on the level of the Godfathers.
Goodfella's and Casino for me are him settling into a happy formula, certainly very entertaining to watch and with some bite but not really inspired to quite the same degree. Actually kind of wished he's stuck with that style more, most of his recent genre pictures have IMHO been pretty uninspired.
Best gangster film ever is Once Upon A Time In America if you ask me, Millers Crossing up there as well with the first two Godfathers.