I would definitely say Catalunya is one of the best tracks for creating base setups. It has a full spectrum of different types of corners, and requires a balanced set up to produce quick lap times.
It should be noted that in pCARS, there is quite a lot of benefit to be gained by having specialized tunes for each specific track. Catalunya is a good place to start, but you'll probably want to tweak that base setup for each track.
Snetterton 200 is a perfect track for setting brake pressure, brake balance, and for adjusting your DECEL and preload differential values, as it features two very tricky curved braking zones.
Other than that, I'd say the main thing is to find a track you are comfortable with and enjoy.
For bespoke race cars like GT3, LMP, and Formula cars, I like Catalunya. For street cars and lower catagory racecars, I quite like Donnington Park National as a test track.
Another good resource to check out is
@dyr_gl youtube page
https://m.youtube.com/channel/UCGaI_TAqZN_RbD7FLyNGThg