I'd say the most humane form of execution, for both the executioners and the subject, is that of the firing squad. Traditionally, one of the rifles is loaded with a full-powdered blank and handed to the members of the squad at random. This means that the members of the squad do not, in fact, know who fired the killing shots. Of course, late 18th - early 19th century firing squads did not always kill their victims with the first volly, musket accuracy being what it is, neccissitating that the supervising commissioned officer finish the deed personally with a point-blank pistol shot. That being said, the likelyhood of surviving a volly fired from modern longarms is absurdly slim at best. Even should the target survive the initial injuries, shock would most likely set in and finish them off.
Those are my thoughts on the matter.
Hmm... I note that firing squad seems to be one of the more popular suggestions...