The three major materials for fixed-blade knife sheaths are injection molded plastic, Kydex, and leather. All three have their merits, with Kydex and leather being the two real contenders, but it really boils down to the following:
Mounting: A sheath that can mount to multiple things (like your belt and your pack strap) can be a major bonus in terms of ease of use and accessibility. One that has a buckle that you can open and close so you don’t need to take off your belt or deconstruct your backpack is even better.
Where you are going: Leather is beautiful, but it doesn’t do so well in the elements. A wet knife sheath won’t dry easily as long as you remain outdoors with it. The universal, and durable, solution is Kydex. That said, if you’re hell-bent on a leather sheath — you can dry it at night on a stone by the fire. Just make sure to take the knife out first!