I’m in favor of the firing squad. Cheap, guaranteed, and volunteers would be easy to find.
While I would agree that it would be just and proper, it would still be litigated to death (pardon the pun) by liberals and others who strongly oppose the death penalty in all cases. I prefer Nitrogen Asphyxiation because it is so much harder to argue against it. Standard atmosphere is approximately 78% nitrogen. Your body doesn't even notice it is dying if there is no CO2 present. You just pass out (fairly quickly actually), and wake up dead. Industrial plants that make heavy use of nitrogen have deal with this effect by instituting all kinds of safeguards, because you simply do not realize you are about to die until it has already happens, and you receive your ultimate reward.