Nurburgring GP/F.
It's got everything you need for a test lap. Straightaway, plenty of turns (at various speeds). Heavy braking.
Which track do you take a new car onto to test it out?
Since GT4 I've always used Nürburgring Nordschleife, and actually thought it would be most people's favourite but I've recently asked a couple of friends and was surprised to learn that they prefer to test cars out on Suzuka.
So where do you take a new car?
I generally use Nurburgring 24hr because you know if brakes and anti-sway bars are ok in the first few turns, and the whole track tests everything. Too low and you spin in "The Carousel"; GP/F can't tell you that. You need more travel to run the whole "Ring" efficiently and more gear, especially if racing more than 1 lap.