Someone my have said this already but I didn't see it...... I'd want the most common defensive round possible to make it easier to find ammo.

I'll bend the rules a little and go with a G23. Tagging along with an extra 9mm conversion barrel/mags. Might as well throw in a 357 sig barrel too. Being able to gather and use two of the most common auto handgun rounds would be useful. The G19/23 size gives the ability to use the mags for both of those as well as the larger 9mm/40 mags. The G26/27 opens that up to the smaller mags as well if someone went that route. Except for 22 rf I don't think there's a better route to go with consideration to finding ammo.

A .357 revolver gives that versatility with the 38's. Pair a .357 revolver with the above and it's lots of options on finding ammo.