I ask new Home inspectors if they were allowed to own just one tool what would it be . Now Remember I said one tool only. I am training my son and he could not give the correct answer can you.
Camera considered like a tool?
If not, my second choice is mesuring tape.
Flashlight
Is this an old grandpa question?
My brain?
From 1 old man to another … I agree with Juan’s suggestion: The Brain.
Well…my brain always follow me… Hihi 
Credit card processor.
As one man said to another…Your Common Sense;)
Don’t outsmart it:D
My blue eyes…
Knowledge.
Ladder
If knowledge isn’t the one your going for… There is no single tool that’s more important then any other. Without the ladder, the flashlight means nothing in the attic, etc,etc. Without knowledge it doesn’t really matter what tools you have.
Marketing skills. If you don’t know how to market your tools are worthless.
Toilet paper.
Charley Bottger?
Not that you’re a tool.
My first thought as well.
Well if brain qualifies… guess it would be the one.
Knowledge, Marketing, etc… all would not happen, or not done or used well without the brain.
Eyesight, blind inspectors miss things.
Vehicle, unless you want to walk to your inspections.
There really isn’t a correct answer to a subjective question. Otherwise, you are just asking him to know what you are thinking. Is it a physical tool or a human asset? That is why he can’t answer and either can any of us. It is your own definition.