A Ruger D/A revolver, with the nod going to a Speed-six. There hasn't been a more reliable handgun in my book and it has some proven power levels going with it.
I've read with interest some of the other replies and made some mental, historical notes from personal experiences. I will add that for me - it's all about reliability first.
As for the Sigs, back-in-the-day they were my very first choice in a defensive pistol "for all seasons and for all reasons". Then I had a Sig P-series get some type of crud in the firing pin channel / safety mechanism that affected the firing sequence ... and it's nowhere near a normal cleaning breakdown to fix that problem. I keep my guns clean and this problem wasn't obvious for me until too late. It stands as my biggest surprise in the hobby ... bar none.
While I dearly love Smith and Wesson revolvers, I've had multiple and memorable issues over the years with too many K and N-frame models to ever call them dead-reliable in such a serious situation. The J-frames are in a class by themselves and if other real-world factors were requested here - they'd be my first choice (five-for-sure).
I've been in proximity to more than one CZ 75-ish pistol problem at ranges over the years to ever want to put such confidence in that one, but a proven Browning Hi-Power or Series 70 1911 Colt would make the grade in the semi-auto class for me.
It seems like any particular makers design has to be around for awhile to properly work-out all the possible bugs that might, some day, prevent it from going bang in the most serious situation the O/P outlined here. IMO, once they make a change (like a Series 80 or internal lock) ... it's right back to ground-zero for a very long while.