My fav is the "one grit" aka a chisel.
Used for almost everything in producing our car.
From preping the buk, producing molds, releasing moulds, surface finishing, quick and dirty hammer, screwdriver, shimmy bar, door stop, resin and paint mixer, shoe horn, tire de beader and scribe.... There is nothing it can't do...besides drive the car!
It's the one essential in any fsae toolbox