What does Brian say is the most important tool for survival?

Brian's most important tool for survival is the hatchet. That should not come as a surprise since Gary Paulsen chose to title the book after that particular tool. As a survival tool, it is critical to Brian's survival because it is such a multipurpose tool. It can cut and hammer as well as act as a weapon of defense. Additionally, the hatchet doesn't require batteries or electricity to be used. It works in the rain, the sun, and any given situation. Brian recognizes the importance of the tool throughout the book, but chapter 16 provides readers with a nice quote about how Brian is mentally and emotionally comforted by the tool itself.

I might be hit but I'm not done. When the light comes I'll start to rebuild. I still have the hatchet and that's all I had in the first place.

That is an amazing tool to be physically helpful as well as mentally and emotionally helpful. If you want to look deeper at the tool itself, the hatchet is a symbolic bridge of Brian's family life. His parents might be divorced, but his mom gave him the hatchet as a tool to use with his dad. It connects Brian to both of his parents, and that is a critical thing for Brian.