The worst would be "I, Mobster" (1958) or "Inside the Mafia" (1959), which are so bad and miscast as to be laughable.
The best MOB film, strictly speaking, is "Goodfellas." GF and II are the best films ever made, imo, but they're not mob films--they're films about a family, with the mob as a defining milieu. I make an analogy with "Gone With the Wind": It isn't a Civil War movie, it's a movie about a family, with the Civil War as the defining milieu.