A .22 bullet to the head is quickest, easiest, and painless.
Twice I've put one in a box with a hole in the top so their head pops out, then shoot at pretty close range with regular-shot 20-gauge shotgun. It takes their head off. Hubby recently just outright shot one. He was so ticked he ignored me and my box idea. I'm just afraid they'll not hold still. He got the head off with one shot.