If a person could take just two pieces of prepper gear with them in order to survive, I might recommend a long lasting survival water filter and then a rugged survival knife!
A survival knife might not provide drinking water or start a fire for you, but it can certainly help in the other areas of survival. You can use a knife in a spear to help hunt for food, and then skin and chop that food for cooking. Your knife also can help you build a survival shelter by cutting paracord, or shaving bark to use in primitive rope. Some of the larger survival knives can even saw through small trees that could be used in constructing a shelter. For these reasons (and all the reasons you can't think of right now), a good survival knife should definately be near the top of your prepper gear list.
The prototypical survival knife is the Swiss Army pocket knife. With so many blades and utility tools stuffed into one compact unit, the Swiss Army knife is one of those pieces of survival gear that should be in every prepper's bug-out bag. Above an beyond the usefulness of an ordinary pocket knife, a Swiss Army knife provides many other tools that you will find a need for.