Outdoorsmen have a love affair with knives, and it's no wonder, because a good knife is probably your single best tool when hiking or camping in the wilderness. From cutting up food to cutting up tinder, from building a shelter to rigging a snare, a good knife is an essential item to many survival techniques. It's important to have the right knife for the right job, so it's important to carefully consider the type of situations that you may find yourself in when choosing a knife. Here are some tips on choosing a knife.
If you can only take one knife, a good pick would typically have a 4" - 6" blade. Any bigger and it becomes near impossible to do delicate tasks; any smaller and it becomes near impossible to do the big chores. If you don't mind the extra weight, you can pack a larger, heavier knife for bigger chores and a smaller knife for more delicate and/or specialized work.
Higher carbon blades, compared to stainless steel blades, can get a sharper edge and stay sharp longer, which can make work much easier; however, when it does lose it's edge, it's a lot harder to re-sharpen. The stainless steel blades are easier to sharpen to a usable (although not razor-sharp) edge, more abuse-resistant and can take more exposure to moisture without rusting and pitting (which makes them better if you'll be using the knife in moist/briny environments). Typically, stainless steel require less maintenance than a higher carbon steel. (Despite the generalities above, modern day sharpeners make it almost as easy to sharpen a high carbon blade, and improvements in production produce stainless steel blades that have very good edge retention; in the end, it comes down to personal preference.)
A grip and/or finger guard can be very important. A slick grip without a finger guard can allow your hand to slip up on the blade, cutting your fingers. While a finger guard will prevent this scenario, a good grip will prevent slippage up the blade, out of your hand, or side to side. While the best scenario is to have both, if you have to choose between the two, I would have to say that a good grip is the most important of the two.
You have to consider what you will be using the knife for. If your skills would allow you to catch game, it's important to have a knife with a good curve to it, to facilitate cleaning out and skinning the animal. If you'll be trekking through an area with a large amount of undergrowth that must be hacked through, a longer blade is necessary. A serrated edge is better for cutting cordage. A blunt tip knife may be useful when rafting so that the raft isn't accidentally punctured. Multitools are perfect for urban survival, but may not as be as useful in the outdoors (unless the local trees are held together with screws and wire). You probably won't find one single knife that will be perfect in every scenario, so it's important to look ahead at the potential scenarios and prepare accordingly.
Lastly, it's of pinnacle importantance to get a well-built knife that will endure a good deal of abuse. It will do you no good to pack a knife miles into the woods just to throw it away because it breaks. A full tang construction (meaning the metal from the blade extends down all the way through the handle) is a must for at least one knife that you carry, and preferably for the knife that will get the most use.
Well, I hope this helps you pick a good one!