Just joking, but me it was the 2002 Ford Explorer and its endless problems with the transmission, the wheels, the windows and everything else that could go wrong.
Just looking at carcomplaints can show you the size of the problem, with 1,500 reports of transmission problems alone.
One of our early car purchases was an Audi A4. I don’t recall the exact model year but it would have been mid 2000s. That car was so dang cool at the time, but unfortunately it was also a nightmare. It was one problem after another and each incident was expensive.