Re: 1 Rifle to rule them all
There is no 1 best weapon, regardless of circumstances. The best weapon of course is the one you have with you (and have ammo for and a system to carry that ammo)....
I am with Flip, that I will no longer partake of the vanilla AR-15 clan. I personally replaced my AR with 2 SKS rifles and could have gone with 3. I have plenty of ammo for it, none of which I reload (although I do have supplies) and being a gas piston type design, it will tolerate adverse conditions better than many an AR when one does not have the time, means or supplies to keep an AR clean and lubed. And I will have at least 1 other person, 2 more for the same money spent as for an AR sending lead at whoever the bad guy is. One man armies do not last long.
If the range is extended, then I have several accurate bolt guns and ammo for them and one would not need nearly as much ammo as with a case spewing autoloader. Elk may need several hits but for most men, 1 hit is all it needs from a hunting rifle. These would not include a 223 unless the carrier was <12 years old.
If the situation is really so bad that I am personally down to one weapon, I would be avoiding direct confrontation and using stealth as much as possible and trying to hang on to the most accurate weapon I have with the best sighting system. I have been abandoned behind enemy lines in my past (until it was noticed that I was missing) and you sure do not want to stick out or attract any attention whatsoever...