I would say firearm (at least for me) since it's effective, quick, and painless if done correctly along with the correct weapon and ammo. Preferably a 12ga shotgun with 00 buckshot loads. As far as making a mess, sure that is inevitable, but if you are able to do it at a remote area like a forest or some other place where there isn't a lot of people, then it would limit the impact of the mess and gruesome scene.