though I use it very little now, it would have to be my two piece Aerostitch. I sold my car and bought one in 1991 and it still works. If they'd only make one with modern materials (read cooler), I'd buy another.
My second choice is a small pair of vice grips, 5" I think. Amazingly useful. I misplaced them for a year once and realized I had clipped them to a fuel line on an old gas tank. Never leaked a drop either.
God. Family. Motorcycles. Guns. Music. Books. Dogs. Beer. Baseball. Work. That about covers it.