Buick Reatta 1988-1991. Could have been a classic but the quality control was terrible. They were unreliable money pits. Any over five years old were nearly sale proof. Other than Credit Unions, few lenders would touch them.
GM earned their decent into bankruptcy. Take the Pontiac Fiero. 1984-1988. Fun cars, utterly unreliable with a reputation for catching fire. The 2nd generation with a V-6 was a great car but too late to undo the damage of the money pits before it. Same money bought you a Toyota MR-2. Great car but my rotund body couldn't slide past the side bolsters.
You can have great engineers and great designers but lose your customers by seeing how damn cheap you can build them. Dodge Intrepid comes to mind.
In 1993 it was miles ahead of the competition. However, the rear seat was terrible. Blasts of cold air coming from the door handle area just right to hit the legs of a child in a car seat. So much noise coming in from the trunk the rear seat passengers couldn't hear the front seat passengers talking.
Ford? Using cheap plastic parts in their automatic transmissions which failed around 60,000 miles.
Still, it made me smile today in Scottsbluff to see that older car carefully maintained and still going.