My worst revolver was my first gun, period. It was a Taurus, bought on the advice of a family friend who was a retired NYPD patrol cop. Timing was a mile off, and the barrel was messed up too, it ripped jackets off bullets, and leaded up to the point that you had to chisel the lead out with a brass rod after only six shots.
Off to FL it went, they fixed the timing, but they said the barrel was "Normal". Called them back, sent it to FL again, and they again denied there was a problem with the gun. When I got it back, the barrel was leaded to hell. I sold it to some goofball who basically was ok with it leading as long as it fired six times. Runners up in revolvers were my Ruger Security Six that seemed to have timing issues that were never fixed. My S&W 686 with the "cheese grater" grips was fine, but those grips had to dealt with. Sandpaper and varnish to the rescue!
In semiautos, the worst was my most expensive gun I ever bought until the last couple of years, a Colt Combat Commander, in SS. Junk. Never should have gone out the factory door. Chatter marks, rough everything. Like the Taurus, it went back twice, and as far as I could tell, the only thing they did to it was scratch it up. I had a local 1911 guy mess with it, and he got it to almost trustworthy status, as long as it was run soaking wet. Runners up include the AMT Harballer Longslide, and the AMT Back-up 45 DAO, which had a trigger that exceeded every pull gauge we put on it.Shoot a mag, your trigger finger was dead tired. Another **** was a Detonics 1911. Those guns and friend's 1911's that had problems but cost as much as a decent used car did back then turned me off them to the point I have never bought another one. My .45 ACP guns include a Sig P220, SAR K2 45, and an S&W 625-1.