Nurburgring - Just an amazing track that has everything
Spa - Eau Rouge, and is challenging
Suzuka - Love this track more and more. To nail the first section perfectly is a huge challenge, and I still kinda always mess up at the Spoon curves, where I lose too much speed I think.
Le Mans - Fast and technical track
Grand Valley Speedway - Then especially the Grand Valley Reverse track from GT3, such beautiful setting with sunset, especially the left-right S-bend right after the last tunnel.
Deep Forest - Fast and technical track, hard to master
From previous GT games:
Special Stage Route 11 - The GT3 version, never played GT2, so dunno what that version is like. Hard track to master, hard to get the perfect lap.
Seattle - Just love racing on that track