Ford RS200
It's a shame that Group B was banned before it had chance to show its full potential as a rally car, because it was probably the best of all of that breed of cars. In principle, anyway.
It was also responsible for one of the funniest things I've seen on Top Gear, when they let Clarkson loose on the test track in one of the original rally cars. He came out of a corner, booted the accelerator and said, 'There's nothing happeni ... SHIT!' as the turbo kicked in, 100bhp suddenly became more like 600 and he spun round in a nice big circle and took the front spoiler off.
All of the Group B cars were impressive. Unlike the rest of them (with the exception of the lovely Lancia 037), the RS 200 also managed to be beautiful. I bet it'd be a stupendous road car ... if terrifying at times!