I would say a strong fixed blade knife crafted with steel that holds a good edge but easy to resharpen in the field. I would say my Cold Steel Trailmaster in Carbon V steel or my Chris Reeve One PIece range knives. Even though the post was titled "If you can only take one thing", I would carry one of these knives in a aftermarket sheath such as a Spec Ops or Eagle sheaths that have an accessory pocket attached to it. Inside that pouch I would carry life boat matches, fishing line, seven strand 550 cord, small bic lighter, a few hooks, a small rolled up plastic bag to carry water, alum foil, ect. So it is like taking just 'one thing' and that would be my knife.