One tool every gearhead should have and why

Its a rare use tool, but a borescope has recently saved me a lot of work when 6 gm injectors fell apart during removal.

I second the power probe,and a circuit tracer to go with it. Together there isnt an open or short circuit you cant find.

I have had a multi-tool on my belt for 20 years. Make sure it has a file,had to cut a padlock off,it saved a 50 mile trip to get bolt cutters.