I don't think anyone motioned the best tool of all - your brain. I understand the intent of the question but thinking clearly and making the right decision during a survival situation can make all the difference. Having the right tools to help you and being proficient with them is also a big plus.

That being said my favorite tool when in the wilderness is my 8’x10’ sil-nylon tarp. It’s light enough to always be packed and can be pitched in minutes.