My firesteel. Or a lighter that if empty could be used as one.

I would go one further and say a good bivy bag.

Ummmm, sure. It looks cools, and it is only $15 from here:
http://www.thinkgeek.com/geektoys/cubegoodies/8cff/
However, it does not make enough noise to be useful for alerting rescuers. Probably not good for much else. But if you want...

For all who have posted, Thank You!

Yes, this is a very vague question. Yes, a tool can be anything that one can use. I live in a very small town and our local Postmaster and his spouse lead a group of Girl Scouts. He and I have exchanged tidbits of info concerning backpacking/camping from time to time. The Postmaster and his spouse along with many other local adults (including myself) have had training outings with his troop. The young ladies asorb the training and info like a thirsty sponge. Things taught to them include primitive firebuilding, shelter building, foraging, primitive water purification, signaling, first aid, orientering etc... All in all these young ladies reguardless of their age are quite proficient in their skills.

One day while I was in the Post Office chatting with the Postmaster, a few girls from his troop came in. The eldest girl asked me this very question. I responded,"my gurkha kukri". I asked her what her choice would be. She responded,"My hatchet".

Twice a year the young ladies are taken into the local national forest or wilderness area for a 3 day outing to put their skills to the test. They are allowed only their knife, multi-tool, whistle, one light source, their personal survival kit (which they have put together themselves)and first-aid kit. Of course, many adults acompany them on these outings for safety purposes.

Interesting.......

In my environment, the biggest risk in a survival sitution would be hypothermia.

If I lost all the gear I carry except one, I would want to make fire so I could stay warm and signal for help.

I would hope I had a good lighter.

I guess I'll cheat a little & say a Multi-tool (LM Wave?) with a firestarting flint attached to the split ring.

If you're taking the sonic screwdriver, then why don't you ask for the entire TARDIS. Personally, I'd rather just take one of the doctor's assistants.

[quote=BrianTexas: If you're taking the sonic screwdriver, then why don't you ask for the entire TARDIS. Personally, I'd rather just take one of the doctor's assistants. [/quote]

Leela!

-Blast, who doesn't see a glass half full or half empty because there are too many dang beautiful women around distracting him...

well, just to stir the pot ...

... the "tool" I would choose (if forced to choose only one) would be a slightly oversize gen-2 goretex parka, knee-length with full hood and drawstrings everywhere ...

... it's practically a shelter in itself when you batten everything down ... it adds to the effectiveness of any clothing you have ... you can add insulation by stuffing in any semi-dry source you come across (grass, reeds, etc.)

... thinking back to my years of solo mountain backpacking, this would be the most critical item for me, hands down

(btw: this is a quite the admission from a total gear nut, knife/axe/multitool accumulator and flashaholic)

Ah, then I want a Nanoman. It's like a Leatherman, but it has a dry-nanite hive built into it, and a controller that can support up to 1024 parts, no more than 3cc's total volume. A two part tool, like a pair of pliers, is two parts; different screwdrivers, manual, are one part each; etc.

For a serious entry, a good dutch oven. You can use that for everything if you have to.