There is no "best" caliber as there are too many variables involved. Picking a carry gun should be decided more on how much concealment is necessary and the ability to use the gun proficiently than caliber. If it requires a bit of work and concentration to hit on the range, it is likely you will do much worse in the pressure cooker of real life. I use one of several guns for carry depending on the situation. When it gets cold enough to require a jacket, I carry a Springield Ultra Carry in 45 ACP as the heavier clothes provide plenty of concealment for a largish gun. It was also my duty gun for a few years so I was very familiar with it. For summer and semi dressy occasions I'll carry a .40 S&W Kel-Tec P-40 as it is a much smaller gun which is easier to conceal. I'll also carry this gun in an ankle or vest holster as a back up gun (BUG) when working. I recently bought a Springfield XD-40 compact which may become my main CC gun as it works the same as my duty guns; I have occasionally found myself forgetting to release the thumb safety on my Ultra Carry when practicing. I sometimes carry either of my present two duty guns, a 4" Service Springfield XD-40 and a 5" Tactical XD-40 but they are a bit longer in the grip which makes concealed carry a bit harder if using inside the waistband (IWB). I recently bought an XD-45 Service model but it too is a bit big for regular concealment usage in my opinion as the grip is even longer than on the .40s. For a number of years I carried an S&W M-66 in 357 mag for both duty and off duty use but a mid to full size revolver can be uncomfortable to carry at times.
I've carried everything from a .380 ACP up to a .45 ACP and feel that anything from 9mm on up with decent bullets will work fine for carry use. I pick 9mm at the minium as there are a number of guns for this cartridge that are as small as the vast majority of .380s so see no real need for the smaller cartridge. Pick the gun that you feel comfortable with that meets your carry needs, stoke it with quality ammunition, practice regularly, and you should be fine.
I've carried everything from a .380 ACP up to a .45 ACP and feel that anything from 9mm on up with decent bullets will work fine for carry use. I pick 9mm at the minium as there are a number of guns for this cartridge that are as small as the vast majority of .380s so see no real need for the smaller cartridge. Pick the gun that you feel comfortable with that meets your carry needs, stoke it with quality ammunition, practice regularly, and you should be fine.