The very first murder I responded to involved a .25. The victim was laying on the ground rapidly departing this world. I saw a little dimple perfectly centered on the bridge of his nose. In viewing the scene it was apparent that he had staggered around for a (very) few seconds before collapsing.

If I could count on making a shot like that every time, I'd carry a .25 or .22. Since I'm not that good I'll carry something that allows for a little less precise placement.