My criteria:
1.) As large a pistol as I can comfortably carry concealed for the specific situation/location.
2.) As high a caliber as I can handle in that size.
If deep concealment isn't necessary, then I'll carry either the Glock 20 (10mm) or Glock 21 (45acp). These are full-sized pistols in just about as large a pistol caliber as you can reasonably get and carry.
As concealment becomes more necessary, then I start to move downward in size. This goes all the way down to numerous .380s that completely disappear in just about any pant pocket. From my experience, if all I can carry is a .380 in the pocket, then it's probably a place that strictly forbids firearms; and therefore I shouldn't be there!
The firearm I carry the most often is a Kahr CW40. It doesn't fit in a pocket, but completely disappears in my waistband with ease. I'd probably prefer the CW45, but the .40 is where the deal was at. I shoot it just fine, but for some, the CW9 would be a better bet for more recoil control.
The firearm options available in the U.S. is amazing! I personally go as big as reasonably possible. The other reason for that is that, in my experience, the smaller the pistol, the more unreliable it can be; or the more upkeep is needed to keep it reliable (ie, weekly or monthly cleanings--at least for me, it's hard to keep up with this).