The 10mm Glock 20
Excellent choice.
Out of the box, the G-20 chambers the most powerful, most versatile autoloading round currently available in a conventionally-sized handgun - one that's reasonably concealable, accepts hi-cap mags, and, because of the polymer, weighs less than an all-steel pistol holding a third less cartridges.
The only thing I did to mine was add night sites. No failures to feed, fire, extract, eject, etc.
Enjoy!