Carlo Gambino was great in mafiosi circles because he made his family the biggest and most profitable. Puzo based Vito's quiet, unassuming, humble but strong nature on Gambino. Also, the wedding day scene of favors was based on the stories of when Gambino would come to Little Italy, the people would say 'Don Carlo' and at a cafe there his men would usher in people who wanted to ask him for help.

Dellacroce would not have made a good boss in my opinion because the old ways of making money (books, numbers,shylocking, etc.) were dying. Dellacroce was a gangster and racketeering was the bigger, safer profits. Unions, Wall Street scams, more white color crimes was the future and it proved itself true (internet gambling, many states having casinos, state lotteries replacing numbers, pay day loans & companies specializing in 'C' and 'D' rated credit, you can have a bankruptcy today and still buy a house 6 months later today!). Castellano was better suited for that. Plus it didn't hurt he was his cousin/brother in law. Carlo's mistake was not preparing Big Paul better or in making Neil a better business man. Neil was a bruiser, not a man with a head for big business. Paul should have consolidated the family after Neil died and been a little more loyal financially and otherwise to the troops. Hindsight is 20/20. He should have seen that bosses do get killed (Joe Columbo).
No one runs a perfect family. He did the best job of any of the other families namesake (Tommy Luchesse, Joe Columbo, Joe Bonanno or Vito Genevesse). Your greatest Presidents never had a perfect tenure in the white house.

All in all, Carlo Gambino did the best. Present day I would have to say The Chin was the best boss of the present day.