In the civilian world, that depends on the individual. I think one should carry what he is most proficient with. All service calibers can work, and they all can fail. No matter which caliber you carry, you should expect a 'failure to stop' and train accordingly. As an LEO for the last 14 years, I've carried 9mm, .40 S&W and .45 ACP duty weapons. In training, we did 'failure to stop' drills with every caliber. The number stamped on your slide is no guarantee of a 'one shot stop.'
Besides, the rest of the world is generally content with the 9mm, including our own Special Forces, the Bristish SAS and Israeli commandos, the latter of which are more far more likely to encounter determined terrorists than we are. Oh, and mostly, they use ball ammo. With the excellent crop of hollowpoint ammo that is available in most of the US, caliber doesn't mean as much as it used to. Find the most reliable weapon you are committed to carrying every day. Then pick the best defensive load that works best in that weapon. Then worry more about training and tactics than about handgun caliber. But what fun is that, right? :laugh: