I haven't read many of the replies. My take on the ideal concealed carry pistol is: 1) You have to have one. 2) It must be affordable. 3) It must be concealable. For many folks that comes down to a .25 or .22.
I'll add: Any gun to be useful must be in the hands of someone who actually shoots and practices with it occasionally. A .22 is far and away more affordable to shoot and infinitely more likely to be practiced with.
And, the biggie in my theory: (drum roll please) No one, even dumb bad guys, wants to be shot with anything regardless of caliber. Most defensive scenarios involve the potential victim defending himself by simply producing a gun and the attacker flees. I believe the mere presence of a gun in the hands of someone willing to use it for legitimate defense is a very effective deterrent. Yes, I'm sure many will disagree with me, that's OK, I respect the other opinions. Three times in my life I have had to produce a gun against the possibility of my life being taken. All three times I produced a .22 pistol and the potential attackers fled quickly. Not a final clincher argument, I agree. But I'm still here to tell the story and that is where I stand with little pistol out of sight.