What happens if you were ever convicted to death penalty, which method would you choose for the execution?

Nitrogen: Your body lacks an oxygen receptor. It easily detects CO2 levels and that causes the discomfort in the usual asphyxiation. If you breath an atmosphere of pure Nitrogen ( or pure argon for that matter), your oxygen level will diminish and you won't feel it. Instead the CO2 level remains normal and you will painlessly go unconscious and die of oxygen lack.