slightly biased
but i have to say the BMC mini!
The way that 75-80% of the car is used for occupant and luggage storage and that it was one of the first proper cars (obviously those BMW 'bubble cars' dont count!) designed with fuel efficiency and practicality in mind decide it for me.
It was designed to serve a purpose in a fuel crisis and managed to last 42 years before being replaced by something which well, doesn't fulfill any of the above!
And all that was done in just over 2 years!