Jomox T-Resonator, it was so noisy when idle, using without a gate was unacceptable. And the idea of the crazy effects matrix seemed better & more rad than it actually sounded. I didn't record thing with it and sold it in about 2 months. I knew something wasn't right in the beginning, but I was in denial, should have returned it while I had the chance. I still have a XBase, which has a little line noise but not near the buzzing that did. I actually like and still use the XBase.
After that, I'd have to say the Korg Nanokey would be second worst, what a hunk o' junk. I had my doubts at the time but bought it anyway because I wanted something small & portable. It was like the quality of a prize you'd get from one of those arcade grabbers (if you're lucky). I could barely get any velocity out of it and I actually thought the keys on my laptop played better, but what do you expect.
Third worst Virus TI, it actually sounded good, but the amount of time wasted getting the TI part to work with Live and failing was unbelievable. Lots of decent takes lost due to crackling, popping & freezing of the USB output. I tried to get it to work for a couple years with no success over several firmware updates. I could have used only the analog outputs but I couldn't stand having something that only half worked, so I lost interest & sold it on eBay.