It depends on what you define as humane. If your definition of humane is to have the one being executed to suffer as little pain as possible, modern lethal injection is ideal. The first drug puts the person to sleep while a second one actually does the killing, thus nothing is felt.
Lethal injection is also the best type if you define humane as "preserving the body in a natural state for a proper burial"
However, if you define humane as minimizing mental anguish prior to the execution, then a bullet to the head in their sleep would probably be best.