Well I have to say Godfather I is my favorite anything. You just can't say enought about the movie. Without its unbelievable success I wonder if the mob genre would have exploded the way it did.I will say this about Goodfellas it gives a look into the real world of the Mafia more than the Godfather I think.
As for worst mob movie I would go and say that Knockaround guys would get the nod.
I'm also going to mention Once Upon a Time in America not b/c I think it is one of the worst mob movies. I think its great but it started so slow and moved unbelieveable slow at times. Still great though.