Brutal:

Savagely violent.
Punishingly hard or uncomfortable.


I don't know if there is a single guy that can be said to be the "most brutal" or more brutal than some others.

The guys that were involved in the William "Action" Jackson murder would be up there. Basically any guys that wanted a guy to suffer rather than just put two in his head.

But maybe the definition could also apply in terms of a total body count. So guys like Roy DeMeo, Nicky Scarfo, Gaspipe Casso, etc. would be up there. To say nothing of the guys back in the day like Anastasia and Genovese.