If I only had one rifle to hunt North America it would be a bolt action 30-06.

However, I do shoot both a 30-06 and a 300 WM. The 300 WM is my favorite elk rifle and I have taken a bunch of elk with it. The little bit of extra range helps, as does the heavier bullet. A broadside elk at 300 yards or less can be cleanly taken with less powerful cartridges, but for a nonresident hunter with limited time the perfect situation does not always occur.

For game lighter than elk I do not think there is much advantage to any magnum cartridge unless you are a long range shooter. Hard recoiling rifles will expose any flaws in a fellow's shooting form and are a bit more expensive to shoot.