Mine would be a tie between the Jack LaLane Juicer, the Cuisinart Rotisserie Oven, and several Bread Machines I've tried out over the years. I was disappointed with all of them.
The juicer was way too damn big for what it did, it was poorly designed so you couldn't even stand a small glass under the spout. It was also a chore to take the whole thing apart to clean it, not to mention hazardous since the blades were like surgically sharp and would easily cut your fingers. It actually came with a separate attachment you needed to screw in or screw off the blade from the unit for cleaning - and it was easy to misplace or lose this attachment. All the parts would practically fill your whole dishwasher, and the plastic parts easily stained permanently from vegetables such as carrots or beets.
The rotisserie oven was a impossible to clean, would overflow with grease, took just as long if not longer to cook than in the oven with little to no difference in the end result. In fact if anything, it was worse because it was a vertical roaster so all the juices flowed to one side of the chicken while the other side was dryer. The extra attachments were useless - who wants to wait an hour for a burger to cook, while it's slowly spinning inside a rotisserie oven? The vertical kebabs caused everything cooking on them to slide off during the cooking process.
The bread machines always produced weirdly shaped loaves of unpleasantly dense heavy bread. I could never get anything close to a loaf of bread from my local bakeries.