This one was mine, a 1998 Mercedes W208. The nice things about it were the cool Mono-Wiper, the engine (supercharged 2,3L M111) and the seats that were perfect for my height. It had a terrible ASR system but came with a LSD, so it was lots of fun in the winter.
The not-so-nice things about it were that the ignition randomly would not recognize the key, a missing catalytic converter, two fist sized rust holes in the front control arms, rust all over the underbody and the absolute worst 6-speed manual transmission I have ever used.
It also had multiple alignments but it didn't take long for the steering-wheel to be off-center again. You could see the tail end "crabbing" in the rear view mirror. There was something very wrong with it, which can probably be attributed to one of the 7 previous owners.
This isn't even counting all minor annoyances like the frameless windows not lowering themselves when you open the doors (that was only a thing after the 1999 model year), the wind noise, the awful interior materials, the broken seat latch in the rear, the trunk lock not working... I could go on for an hour like this.
So anyways, what is the worst car YOU have ever owned?
99 Corolla was a pile of crap. I bought it at 80k on the basis of a report that it was the car that has the fewest MOT failures on record. The doors clanged when I shut them, it felt gutless and cheap generally. I could live with that but the water leak into the passenger footwell had to be eliminated. Spent months searching for the ingress point, finally tracking it down to a poorly sealed door hinge. Around this time I was alarmed by the unusually high oil consumption, it required topping up between services. I asked Toyota and they said it's normal to consume a litre of oil per 1000km 🤣 Needless to say, the engine expired a short time later with failed piston rings on cylinder 3 at 100k.