The Top Gear Test Track, of course.
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.
Top speed testing it is has to be Le Sarthe (no chicanes)
For the whole full bang-test the car till the wheels fall off i put them on the ring
When i unlock Top Gear, i will use that for lap times