There is no definitive answer to this question as everyone experiences pain differently and what may be considered the least painful form of execution for one person may not be for another. However, some methods of execution that have been considered less painful include lethal injection and gas chamber. These methods typically involve a drug being administered that causes the person to fall asleep before they die, which can help to lessen any pain or discomfort experienced. Additionally, electric chair and hanging are also generally considered relatively quick and painless ways to die, although there is still some debate surrounding this. Ultimately, it is impossible to say definitively which method of execution is the least painful as it varies from person to person.