In terms of the most ruthless in the American LCN (not familiar with Riina, but will check him out), it has to be Gaspipe/Casso. This is mostly due to their violence towards their own made guys. The Luccheses seemingly had a relatively long period of internal peace with Gagliano, Lucchese, and Corallo....which made the violence of Vic/Gas seem even worse, by comparison.

Has there ever been a family that whacked, or attempted to whack, so many of their own members- including capos and acting boss? Fat Pete, D'Arco, Facciola, Acceturo, Ricciardi (heck, the entire Jersey faction), Pappadio, etc....
According to Philip Carlo, Gaspipe arranged a scheme to bust out Vic from jail. At the last minute, Amuso changed his mind and decided to stay in jail because he thought he was going to be whacked. And he was the boss of the family!

When Gas/Vic weren't killing their own guys, they were hunting down John Gotti and his top guys. They even had NYC cops (Eppolito, Carracappa), and FBI agent (Doug McCane) and Suffolk County detectives on their payroll....which resulted in more guys being whacked.