Folders are the handiest knives around because they can always just sit in a pocket, waiting to be used. Sheath knives are handier only if you carry a sheath knife on your person all the time, and most of us can not do that. For some ludicrous reason, even some wilderness areas do not allow carry of a sheath knife, while folders are permissible almost everywhere. Everyone should carry a pocket knife in or clipped to their pocket when in the wilderness (or actually, all the time).
Good folders can range from a tiny Case Peanut to a Strider and everything in between. Any size can be useful, but the smallest probably won’t be quite adequate in the field for most people. If a larger knife is carried as well, then any pocket knife should do. Swiss Army knives are some of the most popular models for the outdoors and for good reason. Victorinox and Wenger are the best known brands, but all the well-known models are made by Victorinox. The alox-handled models and the larger models are the most popular among outdoorsmen. The Pioneer Harvester and the Farmer are excellent choices in the smaller alox models. Both have a good blade and a saw, which are the most important options for the woods. Among the larger models, the Rucksack, the Trekker, and the Forester are great choices. All have a larger blade and saw, making them more useful than the smaller models if used as the general use knife. Pretty much all Victorinox models are great quality, but those named are some of the best for the outdoors.
Traditional folder types such as the stockman, trapper, copperhead, peanut, and congress are also good options. The stockman and the trapper are the favorites among these for wilderness use, but all are good. Case, GEC, Boker, and Queen are some companies that make excellent traditional knives. If cheaper options are needed, check out Rough Rider and Steel Warrior. Neither of these companies make knives as good as the others mentioned, but they are very cheap and decent quality.
For a more modern “tactical” style, Benchmade, Spyderco, Cold Steel, and Kershaw make cheap to moderately expensive, but excellent quality, knives. Benchmade is the top of the line as far as these knives go. Their Griptilian is an amazing knife at a little under $100. Mine did everything that I wanted it to do, until I lost it. The Rukus is a huge folder that was recently discontinued, but a great knife. Spyderco also makes good knives. The Delica and Endura are the most popular and best known. The Spyderco Tenacious is a Chines-made knife that is getting a good reputation for a working knife. Kershaw and Cold Steel make slightly cheaper knives, and Kershaw in particular has a great reputation. The Kershaw Tyrade is an excellent knife if you can get it, and the Junkyard Dog II, Needs Work, and Skyline models are also great.
A folder shouldn’t be your only option if possible, but one or more should definitely be part of any outdoor kit!