Lethal injection and Decapitation would go fast, but I think the anticipation would kill me anyway. I can see the electric chair really hurting (doubt I'd die immediately, so that'd be 30 seconds of hell before I'm sent to, well, hell). I'd never do the gas chamber or hanging because I have a semi-morbid fear of dying through asphyxiation, so I guess that leaves the Firing Squad.
Alternatively, if I were the executioner, I'd get the entire firing squad to put like 2 clips full of blanks in their guns before the actual bullets and shout "HA HA! YOU FLINCHED!" every time they fire.