If it was just ONE gun, for all serious hunting/self-defense/tactical/target use, WOW, that would be a hard choice. A DPMS in .260 Rem would probably be the best choice ballistically, but the .308 would win because of the practical availability of ammo. I'd have to give such a gun serious consideration.
BUT, I'm not a huge fan of the AR format though. So I think that perhaps a Mini-30 that's been fully accurized by one of the companies that specialize in them would be what I go with, as the 7.62x39 will do for deer-sized game out to 200yds, maybe a bit more, and with the accuracy of such a rifle, anything on down in size will not be an issue. Obviously, the 7.62x39 has proven itself in self-defense/offensive combat roles, and since the PPC rounds are based on that case, accuracy for target shooting is covered too.
An added bonus is the availability of high capacity magazines for both the AR and the Mini formats.
Now, if it must be a "politically correct" rifle, then it'll be a Marlin 336 in 30/30, again for easy ammo availability, even though I'd prefer a .35 Remington personally. Holds a decent amount of ammo in the mag, doesn't look "tactical" and it's easy to reload on fly without having to take the rifle out of battery, an advantage that mag fed rifles don't have.
I'd stick with a low powered scope, either a fixed 2.5x or 3x, or something that tops out at no more than 6x, or any of the rifles. I'd probably drop the duplex for something with a dot reticle, or perhaps a heavy post, for fast instinctive shooting when needed. The Trijicon 1.25-4x24 with the tritium post would be the ideal choice. Good glass, no batteries needed, and a clearly defined aiming point in even the worst of light.