IMHO a longer sheath knife can be pretty useful in almost any environment. I've had the new Kabar with a synthetic grip and sheath for a while and I wouldn't go on any longer trek without it. It's still compact enough for most tasks but it's also got just enough mass that you can chop small branches for shelter poles and the like. Which is very useful in a situation when you don't carry an axe for some reason, and a lot faster than trying to use a folding saw on a multitool or just whittling away with a small blade. That said, I would personally avoid any blade over 7" and/or serrations.

Another good choice would be a smaller Scandinavian sheath knife, 4" or the like. I bought a laminated blade from Frost and mounted it myself. It's excellent steel, tempered to 61 HRc. Takes and holds a terrific edge. Probably as close to the ultimate survival knife as it gets but obviously too small for heavier work so you really need to complement it with an axe or a dedicated chopping tool on the trail. Anyway, this is a fairly subjective topic because it depends on many factors and above all the local climate and vegetation.