I know it's stretching the original post to include multiple barrels on a single receiver but the T/C Encore with a .17 Fireball barrel for varmints/ predators and a .260 Rem. barrel for big game fits all of my current centerfire needs even though I have other barrels that could sub in like a .223 for the Fireball and a 7mm-08 for the .260. The most versatile 2-barrel combo for game smaller than moose IMHO would be the .223 & .260 or, if bear or moose are a target, I'd replace the .260 with a .308 .
If restricted to one gun with one barrel it would be the .260 because of the wide choice of quality bullets from 100 gr to 160 gr. and the ability to reduce loads or max them.
