Most guys would say "The Godfather I and or II as their favorite. So, lets just leave that as a given.
My fav is probably - "Once Upon a Time In America"
The best show, without doubt for me - "Boardwalk Empire" - HBO. Not only is B.E. the best gangster t.v. entertainment I have enjoyed, I'd say it is the best overall t.v. related entertainment I've ever seen. #1