In answering this question we need to remember that as every year has passed there have been improvements in car design, manufacture and performance. Also in such aspects as maintenance, economy, safety, et cetera. So it is really unfair to compare, say, a 1920 car to a 1970 to a 2010 car.
That said, the greatest car ever produced would probably be the one that made the most difference in the way people's live on mass changed. Which car manufacturer and model of car can we say caused this to happen? What made the world think "Cars, instead of horse-and-buggy?"
The answer, would be, I believe, Henry Ford's T-Model Ford. It swept the world. "Any color they like so long as it's black." is a quote supposedly from Henry Ford. It made no difference, countless numbers were sold. And the model was around for a long, long time.
So, using the criterion of which car brought most changes in the way the world viewed automobiles, the T-Model Ford would come in as 'the greatest.'