The BMW M5 sounds cery accurate, a bit like a Formula 1 car, and the S2000's raspy note is appealing.
As for the American stuff, I like the Shelby Cobra 427's noise. In GT3, it was horrible, but now, I'm happy with it. It sounds just like you'd expect a Cobra to sound like.
Also, the AE86 has that sort of sound you wouldn't get for such a car if it was built today.