The worst revolver I've ever owned was a Taurus Total Titanium .44 Special (this was in the early 80s.) It wouldn't fire a full cylinder of ammo without misfiring, locking up, or malfunctioning in other ways. As for the worst semiauto, it was a Kimber 1911...in fact, I bought three of them, and none of the three would fire 50 rounds without issues. Kimber's CS was as bad as Taurus' CS, except more arrogant.
I do have to qualify my comments...I assume by "worst" the question is regarding function. If it includes aesthetics, I'd have to include the three Hi-Points I bought...one in .45, 9mm, and .380. They did work, but man they are butt-ugly and crudely engineered guns. They do work, though, so Kimber should take a lesson...beauty isn't enough when it comes to a gun!