As far as favorites go, Oldring is one of my most favorites. Most of my best races were at that track, its a very good track.
But for a multiclass series, its a terrible track. Ive done 4-class racing here (ILMS mod), and it was insane. Theres not enough room for all classes to get along, its very hard. Even the 2 class race I was in, GT1 and GT2, was very tight.
I voted for Road America because its much wider, with better braking zones, allowing for more safe passes between the classes. Its not as fun, but for a 6 hour race, we'll have a lot less wreaks.