1. One that is easy to carry -- this depends a lot on the person carrying it, how they plan to carry, and things like the clothes they typically wear.
2. One that is easy to shoot comfortably and accurately -- this depends a lot on the person carrying it, how the gun fits into their hand, training and experience, reaction to recoil, and the caliber they prefer.
3. A reliable handgun - nearly every major manufacturer makes reliable handguns, including sig sauer, glock, springfield armory, walther, HK, S&W, Ruger, and others.
I most often carry a Springfield Armory XDS 9mm. I often carry a Ruger LCP .380 when I am dressed lightly in the summer. And in heavier clothing I sometimes carry a S&W Shield .45 and once in a while a Springfield XDm .40.