Comparing rats and deciding on who was the "biggest" leaves alot of room for interpretation and debate.

IMO, Joe Valachi was important because he was the first to publicly expose LCN, not just to the cops and the Feds, but the public. Most of his information was spoon fed to him, but he opened the door for the rest of the cooperators.

Tommaso Buscetta was important for exposing the Sicilian Commission, named politicians in Italy who were connected to the mafia and exposed the cooperation between NY / Montreal and Sicily / Italy. He testified in the Maxi Trial and the Pizza Connection Trial. He helped put a lot of guys away on both sides of the ocean.

Al D'Arco was devastating to the Lucchese's, and hurt the other NY families. I believe Lenin is correct that he testified in more mob trials than anyone else.

Outside NY:
Angelo Lonardo pretty much nailed the coffin shut on the Cleveland family and provided testimony about the Commission and families outside NY.

Jesse Stoneking helped lock up almost the entire St Louis family, and the Syrian (Jimmy Michael's) and the East Side Gangs (the Berne / Wortman gang).

In Chicago, Robert Cooley was responsible for exposing political and judicial corruption - Pat Marcy and the first ward, Harry Aleman's fixed murder trial. 24 judges, lawyers and cops went down.
Nick and Frank Calabrese insured the Family Secrets trial resulted in convictions.