Depends on how you define "power." Luciano was very influential: he ended the era of the Moustache Petes by arranging for the assassinations of Masseria and Maranzano; set up the Commission, and invited help and counsel from non-Sicilians and even non-Italians. He was really the first modern American Don. But, his reign was short: convicted in 1936, given a double-digit sentence, served 10 years, and was deported, never achieving the same influence again.

Gambino served as Don of the biggest Mafia family for far longer, achieved more wealth than Luciano ever dreamed of, never served a day in prison after he became Don, and, as PB said, died in his own bed--in itself, a significant achievement for any Mafioso. Unlike Luciano, he was relentlessly low-profile, and his influence was often felt, seldom seen. My nod goes to him.