By far,

my worst experience with a car was a BMW E30 I bought back in the 90s. Typical nightmarish occurences:

Water Hoses bursting on high speed.
Constant overheating making me drive with the bonnet open always.

Had a/c but never used it for a day.

Alternator 'cutout' packing up without your knowledge. Car would refuse to start the following morning due to drained battery.

Stabilizer rubber wearing and giving your front wheels 'K' leg.

After changing the engine without much difference, finally sold it for the price of a new car battery. Phew.
Horrible, Horrible car.

Whats the worst car you ever used?