According to Jim Kallstrom former special agent of the FBI the early 70's was the golden age of the mob, when they had the strongest grip on legitimate business in the US. The Gambinos were without a doubt the strongest family in that period, so I think we can fairly certain say that the beginning of the 70's to early 80's was their strongest period..

Then Guiliani and them FEDs had to spoil the awesome party with that damned RICO act