RE: The one item you would not go without...
A good strong hunting partner to pack out my elk. lol lol. On a serious note I would write a checklist and double check it before I left. I was talking to a sporting goods shop owner in Salmon Idaho (central Idaho) and he told me every year he sells 4-5 guns and/or bows to out of staters who fly in to Idaho. And then drive 3 hours to Salmon. And meet their outfitter only to remember (when their outfitter asks what their shooting) that they left their gun or bow at home. He of course sells a lot more of the personal items that people often foget.