Least realistic:
Tony: James did a superb job playing the role, the writers are to blame here. No way does a real life boss do the things he did. I.E. be involved in every racket personally, be a lonashark, kill a capo in his own crew with his bare hands, etc
While I agree that most of the stuff Tony did on that show was something a boss would never do, him killing Ralphie was more spontaneous than anything. It wasn't planned, and remember Ralphie said to him: "What, are you a vegetarian? You eat sausage and beef by the carload", and Tony hit him. One thing lead to another... I mean, it wasn't THAT unrealistic. If Tony planned it on the other hand, that's something a boss would do, but he'd send his soldiers instead.
