Best compact single stack CC gun?
In my experience, for my needs? Combining the reliability, accuracy, balance, ergonomics, overall size/dimensions, felt recoil and controllability I prefer for an off-duty weapon?
In order of preference ...
CS9 & 3913
I have many more rounds fired through my 3913 than my CS9, and used to carry it all the time, but the CS9's slightly smaller overall dimensions have made it the preferable choice in recent years.
I have several other compact & subcompact models, in 9mm, .40 S&W and .45 ACP, including some single column magazine models in .40 S&W and .45 ACP, but none of them suit my own preferences and needs as well as the CS9 & 3913.
The 4513TSW & CS45 are enough larger and thicker to be noticeable, and the 4040PD has more felt recoil and muzzle whip than the 3913, even when the 3913 is being fired with 127gr +P+ T-series loads.
Just from my perspective, the MK40 is too heavy, being much heavier than my similarly sized G27 ... and yet I feel the recoil and muzzle whip much more noticeably in the MK40 than in the G27. Nothing that interests me, and the last fellow I know who bought one as an off-duty weapon developed similar observations. The larger compact K40 has less felt recoil and muzzle rise, but it's even heavier. Might as well as be a full-size service pistol for its weight.
The P239 is an excellent compact pistol, but the 3913/3913TSW is my personal preference because of overall size (especially slimmness), ergonomics, balance and the S&W TDA trigger. Now, as with many other things, there are any number of folks who could use the same factors someone else uses as 'disadvantages', as 'advantages' from their perspective ... and that's to be expected.
Some range time with potential selections would seem to be a prudent idea ...