The Ring is my favorite. It is for the hardcore and very rewarding when you start shaving tens of seconds by driving smoothly and taming the beast. I love it.
Next, I like all the Original courses, even if Polyphony made sure each one has at least 2 tunnels
. Best of the pack is Grand Valley. They should make it for real that one.
From the city courses, Citta is King while Seoul is boring on low powered machines - a blast with F1s and Minoltas though.
From the rally courses, Grand Canyon is my favorite because despite getting technical it has the best flow from the gravel courses. The snow courses I normally don't play at all, such a borefest.
Now the real tracks. As I said nothing compares with the Ring, although I see how its length can be a problem with slow cars - hey even by that you can still enjoy the ride!
The bad ones are the old Fujis. The 2005 though I like because of the last third that's fun. All the rest Japanese tracks are good, except Motegi that has no flow. The American ones are good as racetracks but plain looking. Le Mans is always fun and Monaco is zipping even if the uphill is in reality narrower.
Man, GT4 has great tracks. For me 3/4 of the tracks are really fun to drive on any car.