(As the corollary to cannibal's thread ; - ) )
#1 - I referenced this in my "best purchase" response, but the dubious honor has got to go to the Aerogarden Herb n Serve Vinaigrette Mixer. Sure it whirls things around but the interior is impossible to get clean because you can't disassemble it, as you can do with a blender or food processor. The stock advice, which is to "just whirl soapy water in it", is a joke. You can't remove a film of oil that way. The plastic container holds odors too.
#2 - DeLonghi slow cooker. I bought this after much research and before getting a pressure cooker, thinking it'd be a better way to prepare beans and soups than having the (electric) stovetop running for hours, especially in summer. What a piece of trash this was. It couldn't even bring a liquid to a simmer point after operating for 6 hours (hottest it ever got was 129 degrees and that took over an hour to achieve), and the timer mechanism ran almost 20 minutes slow. It took DeLonghi six days to respond to a customer service inquiry and by that time I had packed up the thing and returned it to the store whence it came.
The only reason I put the Aerogarden thing as the Worst, was that I wasn't able to return it (because it "was used and operates properly"). At least I got my money back for the disappointing DeLonghi.