1st and foremost I'd choose a Lightsabre
All the Swisstool recommendations are inspiring. I ordered a Swisstool X today.
I think that, given how most of the top brands are of sufficiently high quality, the obvious answer will always be: The one with the most tools. That way you have all eventualities covered, and in a survival situation function trumps everything else.
This is a pretty easy question because I was just going through my "INCH" bag (I'm Never Coming Home) and the tool that lives in there is my 2003 Leatherman Wave. Simple, tough, and has almost every tool I would need. And there are no bits to get lost. It's pictured here with the other tools in the bag