If you're worried about exclusivity maybe an RS4 because I don't even see many new model A4s on the street.
But I think the reason why there are so many BMWs on the road is because they're simply well designed and manufactured. What else in the price range will get you such a top notch car? A Z06 is 70k msrp ... RS4 very high 60s i think ... Cayman S ain't cheap either ... GTR who knows. The M slides in ride under all of those prices. Now the RS5 might be something to worry about if that ever happens.