Tough question!
My all-time favorite handgun is the G17 Gen 4. I am convinced that Glock somehow used a model of my hand when they were developing the finger grooves for that model. It really is a perfect fit, although way too large for most carry duties I'd require. Helluva nightstand pistol, though.
In terms of Glocks, I really like the G48 M.O.S. It feels great in the hand, is easy to shoot well and conceals better than the G19. It also is great with a red dot. However, I'd probably end up picking my G43 just because of the ease of concealment. I can pretty much take it anywhere any time of year and not worry about it printing.
All this being said, I spend some 60-80 days every spring/summer/fall actively moving through the prime mountain Grizzly country of Montana, Wyoming and Idaho and I feel far more unsafe doing this than anything else. I have had two extremely close calls with mama Grizz and one very close call with a big boar and I can assure you that during all of these encounters there was no time at all when I would have felt adequately armed with any of the 9mm Glocks. At these distances, I would wish for a 5-shot semi-auto shotgun loaded with buckshot. But, alas, I cannot carry a long gun while doing what I do.
Because of the fear the grizzly bear has instilled in me, I now would (and do) choose the Smith & Wesson M&P 10MM M2.0 with 4" barrel as my only handgun if limited to just one. I carry it in a chest rig while hiking, bushwhacking and fishing and the thumb safety provides an extra level of protection, especially while bushwhacking. The gun fits me quite well and the slightly shorter barrel helps with all-day carry.
I'd only choose this big ol' gun because it is the most appropriate choice for the greatest threats I face. I'd pretty much not want to carry it for anything else.