My first recommendation when asked is usually: a better belt and better holster (or, a gun you can get good holsters for easily). Here's my two carry pieces (of many things in the safe) from earlier today at the range, as it happens.
I mostly, and for the last 15.5 hours, have been carrying (IWB) the one on the left (M&P 9 CORE + X300 + J-Point). Sitting, standing, driving, dog walking, lifting, toddler chasing, everything. No problem.
I carry the small one (Kahr P9...) mostly in a fannypack, when doing stuff where in the waist won't work. More about position than gun size, but moving to the external carry method makes a big gun that much less acceptable. For me, exercise, mostly, but I have done it for not-entirely-permissive workplaces where it was easier to conceal or stuff other places. It is now the least gun I'll want on me if I have a gun. It is small, light, 6+1 but you can get 8 round extended for the reload, and still has a light (LOTS of shootings happen in low light) and a dot so my old eyes can hit things fast, and accurately with the very few rounds I have on tap.
So I add to the system-to-carry requirements: a gun that will work in all environments (light), and which solves the problem (ideally 9, good sights for me). That gives plenty of options, many of the compacts others have listed above do that, or can be made to. TLR-6 light goes on lots of things and if they don't come RDS cut that can be added. I got mine done at Galloway Precision, who specializes in compact gun parts and work like slide cuts.
These days, I probably wouldn't get a Kahr new as they aren't so supported as the finally-small-enough Glocks, or the Shield. When I get older, I expect I'll have to downgrade to a Shield-EZ (or whatever is around then) for recoil reduction and easier manipulation, so happy that's come about.