Well I have had a pretty good, large collection of firearms over many years but two things stand out. First is that I have always been a HUGE fan of used guns and not such a tremendous fan of purchasing new. And secondly... I suppose I have had pretty good luck in the new guns that I have bought.
To be 100% honest, the worst NEW gun I have ever purchased... ummm, I don't think anyone would expect it to be very good. It is a Jennings J-22 pistol. I had shot one that a friend owned way back when I was in high school... so later when I was old enough to purchase and a local gun store was advertising them, I couldn't possibly go wrong at $54.95 plus tax, could I?!
Well... really, no. Considering that price (early 1990's), the pistol is a good lookin' little thing and if you find some ammo it likes... it kind of runs some of the time. Heh. And like most miniature little blowblack pot-metal cheapsters... the barrel is fixed to the frame, so when it does run, it's far more accurate then you are expecting.
Now then, you want a real experience of a brand new, not low in cost, well known gun that was a total disappointment? My buddy bought a brand new Sig Sauer 1911 Match Elite Duo-tone. About 4 or 5 years ago, this was a very good looking adjustable sight "target" pistol that ran somewhere between $1,000-$1,100 but this gun was an absolute problem child.
1) rear Bo-mar style adjustable sight was loose and rattling in the dovetail... it shifted side to side. Didn't fall out of the pistol, but may as well have
2) deep horizontal gouges in the right side of the slide near the muzzle of the pistol... they almost looked like scarring from holster wear except they were UNDER the finish of the pistol. Manufacturing defect, plain and simple.
3) the pistol had an irrationally short chamber and ammo that fed, fired and fully (and accurately) functioned in multiple Ed Brown, Dan Wesson, Les Baer, Smith & Wesson, Ruger, Remington, STI, and Springfield 1911 pistols would NOT chamber in this Sig pistol. Shortening the COAL of the load was the only way to make the pistol run. 230gr Hardball worked, but a bullet of any other shape was not a feeding issue, it was a chambering issue. I never had the tools to measure the chamber... but when I could successfully run the SAME ammo in two dozen other 1911 pistols, I simply concluded that this Sig was out of spec.
It was a very beautiful looking pistol and the front strap checkering was a nice feature. With Hardball, it was an accurate gun as well, but what a damn lemon and it certainly turned me off of Sig pistols, especially their 1911 pistols.