Possibly the nerdiest answer in here, but the Smart Coupe. So much potential, ruined, apparently, by the engine and trans.

Imagine having a lightweight targa top car, with a more interesting drivetrain incorporated.

There are many better cars, not even getting into concepts,but the Smart hurts because the brand in NA has always sucked horribly and the one model that had an ounce of potential was killed before they even tried it here.