Considering the current expectations for warfare...a silenced RUGER 22/45 with the smallest available long eye relief,lighted reticle 4x handgun scope available and let the almighty supply field expedient logistical support.
This is a tough question as I can only have 1. If I had to only have one it would be my most reliable, toughest, badest SW 4506 in .45ACP. You can't break the thing and if I run out of ammo I can still use to as a good billy club until I find more.
Runners up would be a CZ75B for capacity and reliability and a SW 5906 for capacity and reliability...
From a former Marine and a Liberal and 2A supporter
This Ruger P85 MkII because the 'bad guys' would think it was a toy. My favorite, which gets little respect, but is easy to shoot any ammo, is accurate and dead relaible. It would even take out Republicans
Hands-down my 92 for so many reasons with the two most important being 1) I am very capable and accurate with it almost as if it's an extension of myself when shooting 2) I have a plethora of 9mm ammo on hand (followed closely by the fact that it is high capacity which for me is an important variable when addressing the OP scenario).