Pinto and Vega, probably designed to teach people to avoid small cars; beancounters running Detroit at that time believed they could not make money selling small cars.
Corvair Monza. Nader was 100% correct in his assessment of this ultimate lemon.
All French cars, not just Renaults
Rover 3500S (called worst car ever sold in America by Car and Driver)
Triumph Stag ("Swagger up to a Stag, and Stagger Away")
Humber Super Snipe (clutch based on principle of magnetism)
Next year model of Super Snipe (clutch based upon principle of sand)
AMC Marlin (zero-traction "sports car")
AMC Gremlin (how to die in a 5mph crosswind)
Saab 99E with coal-mine pump engine
50s and 60s Volvos, pure ugliness award
80s 6-cylindar Volvos ("Gee, I didn't know ElProducto made cars...")
Early Audis with 4-stroke engines (No law of physics prevents Germans from building trash if they put their minds to it...)
80's model large Mercedes (500E or some such if memory serves, couldn't get from D.C. to Richmond on a full tank of gas)