I have certain track combinations I use depending on where I will generally use a car.
Deep Forest, Forward and Reverse, has several very tricky corners that really put many cars on the edge of their comfort level and it's a great all around track. I use it as a base for tuning most cars of all PP levels. For the 'Nurb, I'd take a car and tune it here first.
For higher PP cars, Road Course Indy really brings out HP advantages/disadvantages and will tell you if your car will do well on hp oriented tracks and bigger, flatter tracks.
For lower PP cars, I generally use Tsukuba as a test track for flat circuits as it has a good sweeper and three hairpins of various tightness and camber.
Grand Valley East is a short but testy little circuit that also has a couple of tricky corners that will push many a tune to the limit, particularly the esses/short straight/right hander/left hander combination. That sector really tests out the limits of your rear grip and how the car will do under heavy braking while changing direction.