Glock 19 - Tried and true. No end to available accessories. Some folks don't like the blocky grip, grip angle, or stock sights.

Sig P320 - Similar size to the Glock 19, but a bit more natural grip angle and less blocky.

Smith & Wesson M&P Compact 2.0. Another option in the same vein.

All of these are striker-fire, compact 9mm handguns that are very reliable.

The compact size is ideal, in my opinion. Small enough it can still be carried concealed, but large enough to be comfortable to shoot.

If you need something a bit more concealable, I would recommend the Sig P365.