Bilster Berg. Probably one of the best test tracks in the world. Extremely challenging, with elevation change at every single corner. It's almost like the Nordschleife compressed to ~4 km. I do have it as a mod track for AC, but I'd love for it to be included as first-party content by some developers. I can somewhat understand why it hasn't been included anywhere though; it's not really a good track for actual racing.
To be fair, the mod was made using CAD data supplied by Tilke's company, so the track surface itself is fairly accurate.