Like the title says, what caliber do you carry most often ( in know lots carry different guns at different times. If this is the case, or you carry multiple guns, just pick what you consider your primary defensive caliber.
I carry Springfield XDsc and Glock 23/ Both are 40 cal. I would like to carry a 45 for the hard hit and less penetration but have not found the size and weight that I am comfortable with as of yet.
When I am feeling lazy I slide a Beretta 32 in a Desantis pocket holster. Any gun is better than no gun.