The Lancia Stratos would be one of my picks as well. Robin mentioned the cost to buy one today, and that's true for a genuine car, but there are some good replicas around. The best replicas have an ex Ferarri Dino engine (like the original), but an Alfa Romeo V6 makes a good substitute.
Other favourite Group 4 cars in no particular order:
Porsche 911SC.
Ferrari 308GTB.
Renault 5 Turbo.
One I would have like to have seen is the Alfa Romeo Alfetta GTV V8, but it was never homologated in Group 4:
http://www.tip.net.au/~mstreet/cars/...eal/gt3000.htm