In this post you can find a list of cars that are eligible in stock form for the Supercar Festival. As it says there are many more cars that can be made eligible by tuning, but they're not listed.
If you're looking for an easy win without spending any money, the Honda NSX-R LM Prototype Road Car '02, which you'll get for free when you win Motegi 8hr Endurance race, will comprehensively annihilate this event without even so much as an oil change or tyre swap in the hands of a moderately competent driver.
You can even reduce the adustable downforce to minimum if you want to grab a couple more A-spec points.