I don't get to pick. We get a Glock 22 (Gen4) as a primary, and Glock 27 as a backup.
If I were picking I'd likely go with a Glock 19 Gen 4. It's small enough to conceal well, but big enough to control well, and unlike the baby Glocks I can still mount a good light. 90% of what I do is plainclothes, but when I need to get into "tactical gear" a 19 would work just fine. I'd want a Glock 26 as a backup.