I am so weird; not one of these posters has mentioned terrain and its effects on traction. In my opinion, it doesn't matter how balanced your suspension is, if when you stumble into the rumble blocks, it throws you into a spin. I use Deep Forest for these reasons:
1. It is short, 2 miles or 1'30 per lap.
2. Almost any car can top end in the opening straight
3. It has the perfect skid pad at the end of the opening straight
4. It has low speed esses (50 to 90mph)
5. it has high speed kinks (100+)
6. It has the best bumpy straight for testing damper/spring adjustments
If you can take a high speed sweeper with reasonable consistency, then you can test your suspension adjustments extremely minutely (within fractions of a second) and its easy. Use the first part of the course to set your basic tune, when it's where you like it, run a lap in practice mode. Now you have a ghost to measure adjustments against. Make a change, take the left sweeper exactly the same as the ghost lap, cross over the very rough terrain with your new adjustment, and as you cross the start/finish your ghost will appear next to you. If you pull away from it, your change gave you more traction, allowing you to hit the start/finish at a higher speed than when you made the ghost. If you get only half a length ahead or behind, it is very obvious, yet half a car length is less than 3/10ths of a second at most racing speeds, can any of us claim to detect that difference through normal testing?
In my opinion, the ghost is the ultimate radar gun for testing minute changes and Deep is the ultimate place to invoke him.