Since you said "one" thing (other than the shells, calls, etc.),
I'd have to say cell phone.
It could be your lifeline should you have an emergency.
Also, carry your cell phone in something waterproof or highly water resistant. I typically carry mine inside a zip-lock bag, in one of my pockets. Be sure you turn it "off" or on "silent" while hunting. And speaking of zip-lock bags, I also put my wallet in one. It keeps my wallet dry and would be very easy to find should it slip out of my pocket.
As to any "one" thing, that's really a tough question, since there are several important things you should be taking with you, such as a good compass, extra water, small emergency kit, etc.
But right up there with my cell phone, would be a GPS (serving much the purpose of a compass), as well as a back-up cell phone and compass I keep in my emergency kit. (Never mind that my primary cell phone also doubles as a GPS, I still want my separate GPS unit.) My emergency kit is in one of those small crush-proof waterproof Pelican boxes. I could take a tumble, maybe break my regular cell phone, but that Pelican box should remain intact.
Depending on how you do it, that back-up cell phone can cost you somewhere around $10 to $20 more a month. I'm using a different carrier with my back-up, and in some areas, it works when the primary doesn't, so it's more than just a "back-up" to me. Am using an LG "Net10" pre-pay which, in one of my hunting areas, it provides 4 bars of "good coverage" while my Verizon simply says "No Service". You can pick up one of these "Net 10" phones at any Dollar General for only $25 and it comes with 300 minutes good for 60 days. Even though you must "add" days and minutes periodically, all the unused minutes carry over.
