9mm
For a long while I carried my Glock 29 every day, because I wanted the "power" of 10mm and all of tha; recoil isn't really a problem and all of that: the truth is, I can shoot my 26 better, faster, and the gun is smaller. I was able to get some 9mm +P DPX and don't feel under gunned at all when I go out. That said, I still have my Glock 20 on my nightstand backing up my shotgun; currently loaded with Critical Duty, but going to find some DPX for it soon; it's a heavy gun plus a surefire x300 on it, so recoil isn't bad.
Glock 17, 20, 26, 27, 29, 30, 33, and 36 (plus a Model 36 and a PD340). It's nice having different calibers, plus I really like me some Glocks, but if I had to run off with one ur would probably be my 26. Thinking of adding some mid sized ones, but it will be hard to pull me from the baby Glocks for carry (a single stack 9mm would be a nice new Glock also...); the two fulls are nightstand guns, 20 on mine 17 on my girlfriends side.
All pistols are weak and horrible fight stoppers; with good ammo, there's next to no difference between handgun calibers.