I actually enjoy these threads as a good mental exercise and everytime we have one of these my choice seems to change slightly depending on a number of factors including what scenario I envision, what time of year and what locale I've decided to bug out to and what others have posted that made sense at the time. I've been to enough war torn and disaster ravaged countries to know better than to think that it couldn't happen here, or it couldn't happen to me, so I do think about these things (probably a little too much).
This is Bladeforums so of course we all have waaaay too many knives (as if there is such a thing! I have 4 folders on me right now, and 4 fixed blades sitting on my desk in front of me that I've been toying with), but Pitdog wanted to know which ONE knife.
For this envisioned scenario I think I would want a larger blade that could do more. A smaller knife is more handy, easier to conceal can be used for a wide variety of chores, etc, but I think I would prefer a larger blade for shelter making and as a good deterant. I would prefer something lighter if possible, ruling out many of the big blades (most Busses, most khukris, and most of the well-known big choppers like a BK9, RC9, and some of our fine custom makers wares, etc.). It needs a sharp tip to aid in gutting and cutting (leaving out most goloks, parangs, bolos, etc.). I would want it blade heavy so that it's a big knife in a small package (probably in the 10-14" range to keep it handier and somewhat concealable). I would want it thinner stock to save weight and to make it a more effective slicer. That leaves some sort of machete. I personally would probably go for a barong because I grew up using them when I lived in The Philippines and the Marianas Islands as a kid, they have a sharp and very usable tip (better than a Latin pattern machete), they're relatively light for the size of them, they're lively for self-defense use, they're typically made of very easy to sharpen and repair leaf spring steel and in the hands of somebody skilled with their use (not me, but I've seen some who really know what they're doing) it can be used for everything from shelter building and firewood prep, to skinnin and gutting. I would prefer one with a lanyard hole and a strong synthetic ergonomic handle made of G-10 or micarta and a 10-12" blade, which probably means custom. So I would have David Farmer make a custom one for me or if Busse Custom would do it I would have them make it for me so that I could have the advantages of INFI. I would then have a good custom kydex rig made for it so that it could be carried in a variety of ways, including on the hip with a dangler, strapped to a pack and on a baldric. Then I would use it daily until I became truly proficient with it to use it for finer woodwork and gutting small fish and skinning small game (not an easy task but very doable by those who know what their doing).
Of course in reality, I'll be carrying a multi-tool, a SAK, a folder or 2 and a variety of fixed blades and maybe even an axe or hatchet for serious chopping. Who needs food when I can distract myself with all those toys? Besides, can you imagine if you had to use one knife if you were stuck in a long term survival situation? I wouldn't want to use a SAK or a Mora to build a serious shelter, if I had to. Possible yes, easy no.