I think Mad Sam DeStefano beats them all in terms of being dangerous. The Gaggi/DeMeo gang were scumbags, but if you are a criminal, you could at least satisfy them if you killed efficiently and brought good money. Sam, on the other hand, did never know even himself what kind of idea would jump into his head the next minute.
As for "equals" to the DeMeo crew in terms of "being bloodthirsty" I would put Jimmy Coonan from the Westies and Tommy Pitera with their habit of cutting up bodies, and Filippo Marchese from Palermo who snorted cocaine and masturbated while watching torture.