These would be guns that weren't only disappointing, they were beyond awful.
For me, it was the EAA Windicator and the Keltec SU-2000. Both had horrible triggers. Shooting the Keltec at 25 yards, I could have made a better grouping by throwing rocks.
Back in the day, Gun Tests shot a Wondernine that was so bad, they showed the tester holding the gun as if to throw it at the target. I don't remember the name of the gun.
So.. what's on your list of worst ever?