So far Roy DeMeo seems to be the popular choice for the most murderous guy...
If were talking about ordering whackings there's many candidates, although he probably didn't personally order all of them, under Castellano's reign an insane amount of people were killed in the name of the Gambino family.
You had DeMeo's crew of course, Gravano, Dellacroce etc etc many many killers, were talking in the hundreds probably, between the years 1976-1985.