If I was trekking around the side of a mountain looking for Billy Goat Gruff, then a 270. If you think you might accidentally encounter something more dangerous, like a moose that might stomp you or a bear that might chew on you, then 30-06.
As it is, for a low lander, in the lower half of North America, considering ease of ammo acquisition, a 223 on the the low end and a 6.5 CM on the high end.
