All of them! Just kidding. I, too, got the NSX/Grand Valley on a first attempt, but it's one of my fav cars on one of my fav tracks. Doesn't look like I'm alone in that.
A couple of the others were done fairly quickly. It was the last two that took some time. Still, though, as far as number of tries, they took less than getting gold on certain license tests (I'm still in the progress of that).