Carlo Gambino, and no question at all. He was the most intelligent mafioso, he always avoided to be heard on wire tapes speaking about dangerous matters. Most of the people and the law enforcement thought for many years Niell Dellacroce was the boss, he never showed himself arrogant and spending his money(and he could afford to do it)all over the world.He avoided to be deported and spent his last days in USA, he made good on his Mafia quote "You have to be like a lion and a fox", he scared his enemies away like a lion and he discovered the traps like a fox.If there is a mistake Carlo Gambino had made is making Paul Castellano the Don over Niell Dellacroce despite his problems with the tax evasion. Ok the boss is the boss, but Niell was the underboss and had the right to be the Don.