None of them is a "favorite" of mine, but some were more successful in their time. I'd say:
--Lepke Buchalter probably was the wealthiest individual and the most powerful within his sphere of influence--apparel trades, trucking, union and other labor rackets of all kinds. He was said to be pulling in $300M/year in the mid-'30s, and was so big and pervasive that the Justice Dept. filed an antitrust suit against him under the Sherman Act for operating "a conspiracy in restraint of trade"--a legal remedy previously used against Robber Barons like J.P.Morgan, John D. Rockefeller, Andrew Carnegie, etc. Of course, he came to a bad end: death in the electric chair, not a very successful climax to his career; and that was after spending two years on the lam.
--Meyer Lansky was a major power (arguably the major power) in gambling in his time. He had vast influence with Mafia Dons all over the country, and real political power in Cuba and Las Vegas. Though arrested numerous times, he served only one short jail sentence in the early '50's, and died peaceably at age 81. But his biographer, Robert Lacey, says Lansky never had the money he was rumored to have: "He was the accountant, not the boss."
--I guess my nod would go to Carlo Gambino, a real .90 caliber pezzanovante, head of the largest Mafia family in America. Often arrested, never served any jail sentence. Lived modestly, never showed anything but a totally enigmatic visage, yet a simple look or nod from him was life and death to many. Truly, the iron fist in the velvet glove.