You'd want something that's comfortable to carry for extended periods, small enough to be hidden without a lot of wardrobe changes, the most powerful round you can fire accurately, and ammunition availabilty. I'm a large person, so what works for me may not be best for some, I carry a Makarov in 9x18mm, a Hungarian PA-63 in 9x18mm or a Walther PP in .32 ACP. I carry each pistol depending on needs, if I want lightweight, yet something to pack a wallop, the PA-63 is what I walk out with, if I need small powerful and not concerned with weight, I'll carry the Makarov, if I'm going someplace where 9x18mm ammo is either too expensive or not availiable, and carrying ammo with me is either not possible or too inconveinient, I'll carry the Walther PP, since .32 ACP can be found just about anywhere.
The 9x18mm, power-wise is above the .380, and just under the 9mm Parabellum, the .32 ACP, albiet aneimic, can be effective in the right loads.
As I said, what works for me, may or may not work for someone else, but all three weapons have a proven track record, two are based on the aforementioned Walther PP, all have been used by a military either officially or unofficially, and all are small, and compact.