With so many sadistic psychos to pick from,this is a tough one. Off the top of my head,I would go with the old Chicago guys like Mad Sam DeStefano,Willie Daddano,and Fifi Buccieri.

Not only were they efficient,but they didn't always give their victims a quick one behind the ear.

Daddano in particular would subject his men to polygraphs if he suspected any disloyalty or snitching. If they failed,he would torture and kill them on the spot.

Honorable Mention:
Tony Spilotro
Albert Anastasia
Roy DeMeo