My bid for the most versatile rifle ever made would have to go to the .30-06 ... hands down. You can take deer to moose to bear as you have a bazillion choices when it comes to ammo.

Another thing to keep in mind is the AVAILABILITY of ammo. I once went to Canada for a bear hunting trip and the airlines lost my luggage. I found a .30-06 to borrow but had no ammo. We were way up in Northern Ontario (above the Arctic Circle) and elebenty-bazillion miles from civilization. We were in a primitive camp at the end of a lake that was 27 miles long and would never see another soul the whole two weeks there. I managed to find a small fish camp at the end of the lake that had the basic supplies ... including .22 and .30-06 ammo! Those were the only calibers they carried. Availability AND variety should be a major factor when choosing a one-does-it-all rifle. There's is no big game in North America that couldn't be taken with an '06.