I voted .38 +P. I'm assuming you are talking about the snubbie since most of the other calibers are primarily pocket gun chamberings. In my experience, it's harder to get bullet expansion from a 2" .38 +p than a 3" standard pressure 9mm and rate the 9mm higher than the .38 snub. I do feel fairly comfortable a snub .38+P or a small 9mm will have some "terminal effect".
.22s, .22lr, .25acp, .32acp, .380acp? I've carried most of them comfortably without feeling comforted. The Beretta 950bs in .25acp is a great pistol, for a .25. It's an 8+1 capacity gun, used to carry it with FMJ. 9 rounds is nice, but how many times would I have to fire to stop the threat? Double tap, triple tap, tap, tap tap tap? Shoot it dry then throw the gun at his head? I do admit it is one of the easiest guns in the world to conceal well.
I see some more milk jugs being ventilated in the future...with mouseguns!