Who do you think did the most damage to La Cosa Nostra Tommasso Buscetta or Sammy "the bull" Gravano?

Buscetta, Sammy The Bull Has More Brains Than Brawn.

Willy, your question assumes an equivalency between the Sicilian Mafia and the American Mafia. There is no equivalency.
Gravano's testimony put John Gotti in prison for life. Most of the damage he did hurt the Gambino Family, not the other American families. But you could argue that Gotti himself--through his degenerate gambling, his mania for publicity, and his long stretches in jail awaiting various trials--already did the major damage to the Gambinos before Gravano turned rat.

Buscetta stimulated a major crackdown on the entire Sicilian Mafia by the Italian government. His testimony had farther reaching consequences.

Buscetta, no doubt.

Turnbull is right, there is no way you can compare these two people. There are too many un-equalilties...