Well, if it really has to be just “one,” I’ll probably bring one of my Glock 17 pistols, loaded with the Buffalo Bore 9mm +P Outdoorsman load that Phil Shoemaker used to defeat that grizzly. I have fired Glocks in some high-stress training scenarios, making some memorably decent hits. I reckon that shot placement and penetration are most important. No, I am not a Glock “fan boy,” but simply practical, being my best option for an ambidextrous service pistol, that is sufficiently “orthopedic” for my aging right hand.
I had wanted to say that I would bring my 5.5” Ruger Blackhawk Bisley, which has .45 Colt and .45 ACP cylinders, but, it has been a long time since my last range session with it, and with my aging hands continuing to surprise me, I am afraid to presume that I can still shoot stout loads with it. I have been shifting to my healthier left hand, for most of my shooting, and my Bisley was better for my right hand, whereas my left hand “prefers” the traditional single-action grip.
In the spirit of the way the original post is worded, I chose from among handguns that I have, at the present time. I have presumed that I can bring a long gun, which would probably be a Benelli M2 autoloading shotgun, for a generic “adventure.” I did not take big game hunting into account.