My vote goes to the Zaporozhets, the air-coolered, rear-engined Ukrainian Corsair. Here's the luxury stretchoviy model. A close second would be the two-stroke East German Trabant. Wenn mein trabbi schneller waere, dann heist er 'galoppi'!
I've actually seen one Trabant on the road: on a Swiss freeway, shortly after the Wall fell. It was keeping up a good speed, so I suspect the owner had a new engine in it.