Need? Well... probably a 30-06. However I own a really good .338 WM that I really like a lot, and a couple .325's too.
I don't have any desire to kill a brown bear, especially since the meat is usually just let lay, and African game holds little interest to me for some reason too, so it comes down to elk and moose. While my 30-06 perfectly capable of those beasts, I would most likely carry something a little bigger.
So I'm going to say .338 Win Mag, with the disclaimer that in North America short of grizzlies and the like, nobody "needs" one...
-jeff
