Mel Tappan wrote the definitive work, Survival Guns, in 1975. His concept of the Working and Defensive classes still holds very true. The gist of it is: Defensive weapons are made to handle sustained firing with fast reloading and field-expedient repairs. If you have to defend your life against an armed aggressor, much easier to do so with defensive designed weapon. Can always hunt with an AR-15 or AR-10, but if you have to fight for your life with a hunting weapon, you may be at a grave disadvantage...
Pretty much it, in a nutshell.
If you handload, and live where dangerous bear are not a concern, an AR-15 rifle with variety of ammunition, or even just heavy match ammunition will serve. Notice, I'm talking RIFLES, not M4gery with 14" barrel and short gas system. 20" 1:7 twist rifle, maybe a heavy or government barrel and a free floated handguard with a good scope or A2 irons for extra durability.
An AR-10 full-length rifle if you live in brown bear or grizzly country. The .308win with a variety of bullets will handle anything.
If you handload, you might consider learning to cast bullets. A variety of cast designs gives you a wide range of mild rounds that can be single-loaded; or cast with wheelweight alloy and spitzer design and see if your rifle will feed from the mag reliably. Might take some work, but some guys do shoot 180gr sp cast from their M1a and AR-10 by magazine feed.
Lots of advantages to the AR-15 and .223. Lots of flexibilities to these AR pattern rifles with quick change upper receivers.
Team up with a 1911 type pistol and a .22 conversion unit and you have a very versatile package. Lots of super accurate and reliable AR-15s out there. Hard to beat the 15 for minimal recoil, accuracy, and wide bullet weight range.