I thought that 1978 Buick Century Special was just "plain" instead of ugly. For ugly, I agree with jinman that the Pontiac Aztek would be the winner, and for trucks, the Chevrolet Avalanche is the hands down winnter.
Even though some of the models were not the best looking automobiles...one exception I think is the AMC Eagle...the single tone 4x4 wagons are a rather handsome vehicle...and very sturdy I might add...the standard curb weight on the Eagle was close to 4k pounds...
I have pulled more 4x4 pick up trucks out of the mud and precarious situations with my Eagle than I can remember...(even one F250)