The Boss is the Boss is the Boss. We're talking about a Mafia Family that has a specific hierarchy just as any other so let's use the proper terminology. There's no street crew bosses or any other fantasy structure people have created over the years.

James Marcello took over after John Monteleone passed in 2001. Al Tornebene was his Acting Boss while he was serving time in prison. John DiFronzo was probably never Boss.