I'll add my $.02 here & say for me, it's definitely the two test for the S license that include both the Viper & Laguna Seca & Ford GT40 @ Seattle.
(I'm not even going to talk about the troubles I'm having with the GT1 @ Cote d'Azure!
)
I've gotten silvers in them both so far but am only mi-nute tenths off from getting gold.....
For me, turn 5 & the corkscrew (LS) are killers for me & that's where I lose most of my time at! :-(
As for Seattle, sad as it may sound, it's turn turn 2 (The first left turn after the mini-chicane by Pit Row entrance is what I'm referring to)
and turn 4....having to somewhat drift the car around the turn & setting it up correctly so as you exit the turn, you're in a good position to handle
crossing the tracks...that usually throw the car to one side or the other depending on how your car hits it!
The rest merely took time to improve on from my original scores (Bronze) when I first purchased GT3....but I'm still mostly silvers for all the test/licenses...