All of them lol. They run similar times from my testing so far (1 track). It's about the cars and how equal they are to each other not the class they would run in real life. The worst part of GT6 is that they didn't give us full grids of car classes or we wouldn't have to do this.
Rad and I were just playing around with the Vipers and dear lord they were fun!
I can say that the jag is fast (so it's HP would be reduced) and the vette is out unless I lower the weight for all in which case the audi would be out.
What I'm trying to do is get race cars running within .5 of each other. right now every car is able to go from 49/51 to 51/49 except the audi.
Any help any one wants to offer (even if it's while you test your current GT1 specced car) would be greatly appreciated running laps by myself get tedious