I've liked all of the race circuits that have appeared in the GT series, though I do prefer Grand Valley and Deep Forest over the rest just for the fun of driving them. I haven't really liked the night/street courses all too much, I've won them easily enough, I just don't find them particularly fun, except for maybe the odd one like those coastal/hilly road races in GT4.
When I think of a "Gran Turismo" track, I will always think of Autumn Ring (mostly because it was the first race of the Sunday Cup in GT1) though, so thats what gets my vote.