My top 10:
1. Nürburgring: narrow, fast, long, most epic track in the Gran Turismo series
2. Côte d'Azur: difficult to overtake, street circuit, legendary F1 track
3. La Sarthe I/II: long, extremely fast and I really like the flow of the track
4. SSR 11: technically challenging, fun to race
5. Infinion Sports Car Course: has some very specific corners. especially the first section of the track is 'yummie'
6. Citta di Aria: very narrow street course, difficult to overtake
7. Trial Mountain: just because

8. London Reverse: by far my favourite Prologue track. has much fewer good overtaking spots than London Normal, which can make for a challenging race.
9. Opera Paris: don't really know why, but I spent a lot of time on it in GT4
10. Hong Kong: despised by many, but loved by me. Has some challenging corner sections, and I hope for a return in GT5.