Puma PPS/50 22. It looked cool, and was fun when it worked, but it had a lot of problems in a very short timeframe. I had quite a few FTF's, and after maybe 150 rounds the drum mag bound up completely. I tried to send it in for warranty repair, which was difficult because the distributor, Legacy Sports, refused to give their FFL to UPS or any other shipping company! I ended up having the shop send it in on their FFL, and it came back with a new magazine. Next time out I was still having FTF's, then the trigger jammed up. It turned out to be a bur on the disconnector which wasn't allowing the hammer to drop. A local gunsmith took care of that, and I opened up the feed ramp to try to improve feeding, but next time out the firing pin broke in half. I wrote the factory directly and they sent me a free replacement. I installed it, fired a few rounds to make sure it worked, and promptly sold it before anything else had a chance to break.
This one is going to raise a few eyebrows, but honorable mention (because it was a gift and not a purchase) goes to a Ruger Single Six convertible. It's not great for me ergonomically and I hate how the loading gate and ejector rod don't line up with the clicks, which makes the tedious process of loading and unloading a SA even worse. I could probably get used to that eventually, but there are quality issues as well. Brand new out of the box, the trigger and hammer were both very gritty. I took it apart and found some rather rough parts but not a molecule of oil. A little filing and oil improved things significantly, and maybe they would have smoothed out on their own with time, but my first impressions of quality weren't great. By far the biggest problem is the chambers, which are so tight that after firing 3 or 4 cylinders I can no longer fully seat the cartridges. I'm pretty meticulous about cleaning and maintaining my guns, but even I have little use for a gun that will only go 24 rounds between cleanings.