If I were only allowed one, I think my .257 Roberts would be the one. I almost listed .308, though. But, I don't hunt anything bigger than hogs and deer.
I can't live with just one, though. I mean, how am I supposed to hunt squirrel with a .257 Roberts? I guess the vote don't include shotguns and handguns. But, I just don't think I could do it with just one rifle. The closest thing I have that would come to all around capable is my .357 magnum carbine. I have light .38 loads that can double for .22 and heavy magnum loads for it that shoot strong enough for deer/hog out to 100 yards, so perhaps I should ammend my choice to the .357 magnum. I think I could get along better with it, though it's not a long range gun and I'd be SOL on some of the shots I've made out west, and it's not exactly bench rest accurate.
I do handload, so I guess if I were FORCED to live with one rifle, I could delve into the world of squib loads for whatever I chose for small game. The .257 might be a good squirrel rifle with a cast bullet of 100 grains moving abut 900 fps at the muzzle. Don't know if I could make that load happen, though. I might make the .308 do that, too, who knows. Thankfully, I'm allowed more than one rifle for more than one application.
I ain't interested in "shf" or military applications or killing people. I'm a hunter, plain and simple, not a mall ninja or survivalist wanting full auto firepower, thank you. So, my choice would be a manual action for versatility, not a semi auto.