Originally I would have said Lucky, but taking in to account the two lengths of their rule, Gambino's twenty years is truly an achievement. But again no one dared challenge Luciano whereas Gambino while always emerging victorious had to deal with the Bonanno war whom could have won if not for a heart attack and thus his retirement to AZ, the Colombo fiasco where again if not for Profachi's death and Magliocco being the only one left giving up could have ended different. It just seemed for the short time Lucky was boss, his will was unchallenged and there was peace throughout the mob. Even Anastasia who was itching to take out Mangano for years waited until Lucky was deported. For simplifying things, say

Lucky in 1935 right before his imprisionment. and Carlo in 1975 right before his death