It looks like we have a mix of big bore suggestions (.45cal and larger), as well as medium bore suggestions. In my opinion, the best medium bores for North America would be the .338 Win. mag, the .340 Weatherby, or the .375 H&H. I would pick one of the first two because there's plenty of situations where longer shots will have to be made.

For a big bore for North America, maybe the .458 Lott.