Ruger 10/22--the magazine would fall out every third shot--sold at a loss with full disclosure.
Tokarov TT, safety would engage every few rounds. Traded it on a malfunctioning Buckmark that, with a new recoil spring and pad, was a sweet shooter that functioned flawlessly.
I would say that I probably disliked my first gun the most, a Ruger MkIII 22/45, but it wasn't my worst.
Worst gun I currently own . . . a non-functioning 100 year old Marlin 1898 shotgun, that I intend to fix and reblue at some point. Overpriced at $50, I see it as a project to keep me busy.