Worst gun I ever owned was a Ruger Mini-14. Horribly inaccurate with any kind of ammo. It wouldn't shoot a group, you could barely call it a pattern. It went away in a trade many years ago.
Worst one I still own is probably a pristine S&W M27 made in the 1950's. Reason it's pristine is probably because the flash gap is so tight (one thousandth of an inch) that it starts binding up after firing one or two cylinders of ammo, so it has had very few rounds through it. I really ought to get it opened up one day.