The worst sex toy that I ever purchased was a bullet a few years back. It was so weak!
Waterproof Wall Banger. My first mistake was ordering it from "another" site. (before I found EF). The suction is worthless, the power weak, and the prongs (it is modeled after a Rabbit) are so pokey, they acutally hurt. The only thing it's good for is an anal dildo basically. I wasted $30 on a piece of junk.
Some plastic vibrating anal "beads" from Doc Johnson that were horrible in every way possible -- painful sharp seam that had to be sanded, buzzy loud vibration, unsafe cord situation, etc. They made their way to the trash pretty fast, and the electronics from the controller are now a dimmer switch on a lamp.
A jelly vibe from Lovers was probably the worst, just because of the material and the fact that it broke after a day and it was a replacement for the same toy that had also broken in a short period of time.