For me, in no particular order, would be:
Autodrome Laggo Maggiore: Every aspect of this track is simply exceptional, from the overall look to the flow and enjoyment of nailing a lap. Easy to learn but hard to master, with an iconic hairpin and capable of producing some of the best racing i've ever had in Sport mode while appealing to all types of cars, it puts to shame even some real-life tracks. Overall, a classic in every aspect.
Dragon Trail Seaside: This one is among the most popular GT Sport originals and for a good reason. An overall fast circuit, on which to be fast you need to know the limits of your car to perfection and take advantage of every inch of tarmac and kerb. And no corner encapsulates better the spirit of Dragon Trail Seaside than the Chicane of Death: Precision, courage and a tad bit of luck are neccesary to nail this corner. Get all of them right, and you gain several tenths. Get one of them wrong and your race will be over.
Tokyo Expressway-East Loop: Take the speed of Dragon Trail Seaside, the narrowness of the Monaco GP circuit and you have the Tokyo Expressway East Loop. Being a high-speed track, it requires precision, like Seaside. However, instead of risk, the East Loop rewards patience and planning ahead of your opponent, since the narrow walls are nothing but merciful to those who overestimate their abilities. Overtaking here is also much harder, since the only real chances you have are the first and last corners and the long straight that connects them. And even then, at the straight, the car you just overtook can do just the same, since it is long enough to allow that switchback. No place that you gain is secure and every overtake must be calculated and done at the right time. Patience and planning summarize racing in the East Loop of Tokyo Expressway.