The worse car I've ever owned was a Vauxhall Astra Belmont. Whilst there was nothing especially wrong with this example (they were all dire), it was bloody awful. Corners - nah, stopping - nah, acceleration (on the original engine) - oh yes, come to daddy

Replacement engine, after the original's valves and pistons had a little chit-chat at 70mph in 1st gear, was average, and thus showed the car for what it really was - shite! Only had it 3 months, but did around 15k in it.
Still, I replaced it with another Astra, a GTE this time. Still a poor car, but brilliant spec for the money (joyrider era), and a stunningly good engine. Again, appallingly bad chassis went several embarrassing situations with less powerful cars at the lights.
I also had a Vauxhall Nova, that I famously 'found' in the garage. Great little car to be honest, but has to be in my list of 'worse' cars simply because I couldn't get the seat set right, no matter what I tried. I always found it difficult to drive, because I couldn't get the basics right (seat, mirrors etc) - yet my brother's 2 (newer) Novas never gave me that problem. Replaced the Nova with the Rover 25 that I still have in April 2000...