Minimum caliber you would CCW

[electrohawk]

It used to be .380
Then 9mm in a Kahr
Then the P365 came around. Now no excuse that I can't fit a pistol with 10+1 and a 15 round reload in my pockets.

 

[Abbey]

I usually carry a 9mm but carried a j-frame for years and never felt inadequate with .38spl+p

 

[the1_roadrunner]

I always carry .45 acp but 9mm would be my minimum.

 

[SecretV]

Obviously anything is better than nothing, but I'm assuming you mean daily use not shtf.
9mm because of price, capacity, availability, and commonality. Lots a 9mm guns out there, lots of 9mm out there, price is generally lower.