Scarpa. Hands down. He burnt 3 bosses, provided the PC for the Commission case, and passed off information for more than 30 years. Most of Valachi's info and testimony came from Greg Scarpa's 302s that had been going to Hoover a few years before Valachi was on radar.

If I remember correctly, Scarpa was THE FIRST informant EVER for the FBI. Guys had rolled and testified in cases before connected to OC, obviously, but usually only in cases that they were involved in, and on their co-dee's. Scarpa's intel is actually what got Hoover to actually admit that there was indeed a "LA" Cosa Nostra operating in America.