It's not vintage, but my least favorite machine is a White Jeans Machine made in the 1990s, I think. Piece of junk! I had so much frustration using it that I haven't touched it for years. And that is even though it is working according to specs. I should sell it.
For vintage machines, I have a Singer 127 treadle head that has given it's all and needs to be retired and replaced, and I'm not crazy about the old Elnas, either. One other machine is the Pfaff 1171. I still use it for bindings, but that darn machine is so hard to set up that I've named her "Mrs. Fussy Pfaff". I have a Pfaff 130 that I love, but don't think I'd buy a new Pfaff, especially if Singer makes them.