What ever will end it quickly, and not present a high risk of injury to yourself. (had a few "kickers" in my time, best to shoot again in that situation) BUT, I seem to remember that some states have some strange laws on what is considered "proper" ways to finish an animal. (probably put on the books by Humane Societypettitions[:'(] )

So, one that's just laying there and no threat to you physically,
a stab to the jugular (cutting makes too much a mess to the carcas)
or "pith" them (small narrow knife stabed into the spot between the base of the skull and the vertibre, sometimes up into the skull)
Or, if it's kicking and struggling,
One clean shot to the chest. (head shots are WAY too messy, especially close range)