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?
My first car, a 1976 Chevy Malibu, that I inherited from my Grandparents. It looked awful. Was amazingly underpowered. Handled poorly. But my biggest complaint was the butterfly valve on the carburetor. When the temperature got "cold" the valve did not contract as much as the housing around it. Consequently, the valve would stick causing a bad air-fuel ratio that would make the engine stall. I put cold in quotes above because I lived in Houston at the time and the weather is never actually cold. GM took 700 lbs of weight out of the Malibu in the two following years without making the car any smaller. The huge V8 engine only developed a nominal 160 HP (or something like that) and only managed 12mpg. The plastic pieces on the interior cracked and flaked off.
Modern cars are so much nicer and more reliable. I almost feel grateful for that heap because it helps me appreciate the improvements we have achieved.