Hard to know for sure, Rob, as these guys weren't the type to file income tax returns--in fact, tax evasion was the number one reason for convictions before RICO. However, some authors say the wealthiest gangster ever was Louis (Lepke) Bucholter, boss of the garment rackets and Murder Inc. in the '30's, whose income they put at $300 million per year (vs. a reputed $80 million for Al Capone in his peak year, 1927). Lepke completely controlled the furriers and other garment unions, owned several of the biggest clothing manufacturing firms, bossed the bakers' and the pocketbook makers' unions, was a partner in the movie projectionists' union, and had a toehold in the Hollywood studios. He also derived a lot of income from Murder Inc. All his money availed him naught: he was the only gang boss to die in the electric chair.