Well, worst vintage computer for me is subjective. Depends on the category of what broke or what went wrong and why.
Worst PC of all time to me is the Dell 5150 laptop from the mid 2000's - I know, not vintage, but I swear more than half the calls I got on those I had to replace the screen, motherboard, and hard disk (basically the whole computer) about all that was left was the bottom plastic - or as the call center called it "de bottom plasteek" - and what's worse is the parts distributors for Dell PUSHED me to get that "bottom plasteek" no matter what was wrong because I swear they had an entire 4th of the warehouse probably overfilled with 5150 "bottom plasteeks".
Worst vintage systems, probably Packard Bell, with Dell being second runner up (325SX) - mostly for CMOS Battery leakage issues, corroding motherboard traces, and other terrible maladies. Pulled a Packard Bell Legend 843+ out of a dumpster - the rare EARLY LPX tower they made (with the 70's sunglasses tint swing out front door). What behooves me is my GEM 286 just turned 30 years old, and that thing STILL has the original CMOS battery - same battery type as those Packard Bells - the shrinkwrapped pack of what looks like coin cells soldered to the motherboard - and it STILL has a charge, and STILL works, and zero leakage. These Packard Bells were barely 10 years old at the time (circa 2001-2003). I dunno if the make of the batteries are the same but whatever Octek used was one hell of a battery - GEM must have really known some good motherboard suppliers. Same issue with the Dell but it had one of those black bricks with the wires coming out of it and the genius who put it together must have thought hanging it right over the keyboard address lines was a good idea. I put mine on the floor of the case behind the floppy drives. Far less work to scuff off the crap from the bottom of the case than the scuff it off a motherboard and re-solder hair-thin traces of copper afterward.
Worst for parts availiblity was the Compaq Portable 486c - I hunted for SIX YEARS to find a 128902-001 color 640x480 10" LCD panel for it only to come up short and give up, selling it to someone here. Really a shame as it would have made an excellent retro-gaming rig had the LCD not been cracked. And the worst part is now I get all these weird e-mails from Chinese industrial suppliers now because they would claim to have said screen used. Went to about five different parts places who advertised having it online at a premium and they did not have it in stock. My two Deskpros though - man, I could just dig up Drive Rails, Tadiran batteries, and weird connectors for days for those things. Plus I managed to get the 286 version to support an 810MB HDD on the naitive BIOS but even putting a Seagate 540M in the 486c made it act like a confused robot....
I could add more, but I got stuff to do....maybe I'll come back and reminisce my crazy retro-PC nightmare adventures later.