i like to down them and my mate always teabags them which is annoying because i get the bad rep and not him (we are the same character).
Chainsaws are an old favorite of mine although nowadays i like to down them with an AR from a Longshot, down them when they get up and just before they press the A button to roll away aim a little higher and their cranium becomes part of the atmosphere.
But the best way to kill someone is from a head shot nothing beats a good Bullet to the Bonce.