One pistol .... tough decision; .45 ACP or 9mm for a caliber choice based on current technology and historical availability ... I'll go with a .45.
Pistol: My choice: a 1911. In particular, Sig's superb 1911 RCS. Sporting an "Officer's Model" grip length but with a 7 round mag, and checkering fore and aft on said grip, it also has a match tuned 4" bbl., throated and relieved for reliability. Novak sights, and a combat match trigger that breaks at 4 lbs round out the picture. All I need/want in a carry piece; I've never found a better 1911. On the range, it's as accurate as my tuned Gold Cup, but it's light weight make it a far better choice for personal CC. Plus, I do have a Sig manuf.'d .22 LR upper for it that allows low cost practice.
Or .... if a more durable, long wearing model is desired, and the add'l weight isn't a problem; I'd vote for either Colt's Combat Commander or Ruger's SR1911 CMD. While not as accurate as the above Sig 1911, both will still hold 3" gps at 25 yds. Too, I have 9mm, .38 Super and .22 LR uppers that fit both guns, making them in fact, 4-caliber 1911's. This may be cheating as to the original intent of the OP's thread specifying "one pistol" but.....it's a sweet combination that works for me.
Pic's below. Upper is the Sig 1911 RCS. The middle one is my Ruger 1911 CMD with the Colt .38 Super slide mounted, and lastly, a shot of that same Sig 1911 in an Avenger type high-rise OWB holster of my own construction.
Best regards & YMMv, Rod