Big and slow.
Definitely a .45.
In fact the second rule of handgunning, in my humble opinion as a retired peace officer, and s.w.a.t team member here in Texas, is that if it don't start with a "4" then stay away from it.
The first rule of handgunning is, of course, have a gun.
You know with all the discourse about knockdown ability etc, people often overlook a couple of important facts.
First, shot placement is FAR more important than caliber.
Secondly: In order for potential energy (muzzle velocity, bullet speed, bullet weight, etc.) to be translated to kinetic energy (the ability to immobilize or incapacitate, knockdown, or stop, the bullet must STAY in the target. That's why I've always preferred big bore, slow moving bullets. A bullet like a high speed 9mm often loses effectiveness by taking most of it's energy with it after it exits the target, thus also potentially endangering an innocent person.
Big and slow, that's the way to do it.
Anthony