It's the one needed to get the job done at the moment. This is actually an impossible question to answer. Some projects require me to use every tool in my shop, while others require nothing more than a hammer and a couple of nails. It also sort of depends on your point of view. Where I work, everyone thinks theirs is the the most important job in the company. Whether it is engineering, manufacturing, sales or finance, everyone can rightly say that without them the company could not function. The truth is that it takes everyone for the company to function properly. I think the same is true when it comes to which tool is the most important. Everything is important, if you need it and don't have it, you are stuck. One could argue that without a pencil you could not get the project done, so that is the most valuable tool in the shop.
Larry J Browning
There are 10 kinds of people in this world; Those who understand binary and those who don't.