I see that somebody already mentioned the 1981 Chevy Citation, and that would have been my pick for my worst-ever car...if not for the 1974 Volvo that I owned prior to the Citation.
Yes, the Citation did need to be repainted 3 times in order to actually cover all of the primer that was showing through the original paint, and yes, the manual trans did need to be rebuilt twice, but the Volvo was far worse. That Volvo, bought new and maintained better than the mfr specified, treated me to the following situations:
Heater inoperative upon delivery. Three dealer visits later, they finally discovered that the temp control knob on the dashboard had never been connected to the heat control valve on the heater core. Great assembly quality!
Two weeks after delivery, all dashboard lights remain lit up after ignition is shut off. Faulty ground is found and corrected (supposedly). More evidence regarding assembly quality.
Shortly after buying the car, I heard a fearsome noise under the hood as I sat at a traffic light. Upon opening the hood, I find that two of the bolts holding the A/C compressor had sheared off, and the compressor is more or less migrating around on top of the engine. The quality of materials is definitely suspect.
During freezing conditions, all lights on the car operate at only partial power until engine fully warms up. Simultaneously, all dashboard instruments are inoperative and engine has reduced power until engine fully warms up. Dealer was never able to resolve this particular ground problem. This car had so many electrical gremlins that I actually can't even remember all of them.
Engine began burning oil at ~60k miles, and trans began leaking at ~ the same mileage. By 70k miles, it was necessary to add 1 qt of trans fluid every 600 miles. Indy trans shop finally refunded my money when they couldn't remedy the problem after 3 attempts.
The "constant injection" system was a piece of crap, and needed constant adjustment. Idle was extremely rough.
I needed to get the timing adjusted twice a year.
Explanation: In order for the engine to actually have sufficient power, the timing had to be adjusted in such a way that the car could not pass the annual state emissions test. So, I would have to get the timing adjusted in one way for the emissions test, and then return to the mechanic for readjustment of the timing in order to be able to actually operate the car in a semi-normal manner.
All exterior paint was "chalked" by the second year of ownership.
The electric fuel pump burned out approximately once each year.
If Volvo had stood up and made good on the incredible problems with this car, I might have considered another one. However, the company's cavalier attitude toward the rolling disaster that they sold to me resulted in my attitude of, "never again" regarding that make of vehicle.