15: Chevrolet Vega
Ford had a disastrous time selling the Pinto, and across the way, GM had exactly as incapable of a time with the Vega. Interior quality for the Vega was awful. But aside from that, there was worse (via Macs Motor). The reliability of the Vega was almost laughable. These things would break down a ton and repairs were problematic. Gas mileage was the mission of the Vega, in exchange for lackluster performance.
The Vega is also prone to frame and body rust, a problem many cars from the same era faced. Vega owners looking for cheap transportation got more than they could chew with this lemon. Sales of the Vega were poor, and the car was eventually replaced by a new model. Chevy suffered from many terrible car designs in the years that ensued. The brand never did get the compact car all the way right, unlike how its foreign rivals did.