Honestly it doesnt matter much what I get caught out with, as long as I have something it will be used to the best of my ability. That said.. I pick an AR in 5.56 because-

Excellent ammo availability
Proven durable and reliable
Lightweight and low recoil, I know I can carry it all day and shoot it as much as is needed
smaller lighter ammo, can carry more
Can reload in the field with a hand press
Breaks down and conceals in a backpack, discreet bag, etc.

.308 is not a bad choice, but the heavier weight and recoil make it a second choice
.22LR/22Magnum is a great choice, but cannot reload either round