Pistone is the only FBI agent who in my opinion really accomplished a successful undercover job on the mafia.
Pistone did a good job, but he mostly focused on Sonny Blacks crew only and the main defendant didn't even live to the trial. I think Jack Garcia did more harm to the Gambino family than Pistone did to the Bonannos, because he got the acting boss (Squitieri), and 2 capos (DePalma and Megale), apart from a larger number of defendants.
On the other hand, Pistone worked during the period when the mafia was more powerful, so was in a more dangerous situation though...