I'd love to say my santoku, but knives are utensils not gadgets. I'd have to say the best gadget is my KitchenAid food processor. Love it and use it almost daily. Waste? The electric skillet probably. We never use it, it was my husband's before we got married and in almost 9 years I've never seen it used.
(I've gotten rid of a mandolin, electric slicer, electric knife, and a rice cooker, none of them worked right.)