I'm not sure he was the highest (he was a soldier I believe), but Brooklyn's "Freddy the Sidge" Santantonio also did a lot of damage during the time he was cooperating. Until they put a couple of slugs in him. He gave the FBI lists of guys who were members, what crews they belonged to, etc. Similar to Scarpa.

The difference being that the mob uncovered Santanotnio and ended his chatting. So his use to the FBi was short lived compared to the 30+ years Scarpa was spilling his guts.

To our knowledge, Scarpa and Santantonio were the two most important rats in Brooklyn during that era. I'm sure there were others, in Brooklyn and the other bores who were doing the same to varying degrees.