How about a large cat. When you get hungry a week or two in, you can eat the cat and use the fur to make a garment. Until then, you have some self-cleaning and reusable toilet paper.
I'd take A sock.
I know that the show "Naked and Afraid" has been discussed here. I recently discovered the show, and, like others, have found it lacking in a variety of respects. For me, however, it does suggest a thought exercise. If you were selected for this show and could truly pick any ONE item, without knowing what locale you'd be dumped into, what would it be? Let's forget the partner, and assume you got a knife as a freebee (as a knife is often picked by the partner).
I find myself thinking about a roll of clear plastic. Shelter, solar still, make a cup for solar/UV purification, keep you dry, rudimentary clothes, etc. It has a lot of uses.
(As an aside, I've also wondered why none of the people has unraveled their bag to make cordage for snares or to sew a garment. But please don't get sidetracked by ranting on the show!)
So, other than the knife, what would be your ONE item? (No kits, condos, vehicles, resort hotels, etc., please.)