While we're talking more mundane cars, the 5th gen Volkswagen Passat was quite impressive, Cd of 0.27. Even better than the smooth Audi 100's 0.30.
Handsome looking thing too, and a nice simple design. One of those cars that shows that getting the details right is almost as important as having a particular body shape or roof angle. Also a feature of Renault's 2005 Fluence concept, that's now become the Laguna Coupe, which has a drag coefficient of 0.30.
Anything below 0.30 isn't to be sniffed at in a road car, I reckon. The lower the better though, obviously.