I found that the Subaur vs Impreza races are good, having unlocked Impreza Forza edition and kitting it up, this seems excellent for class A and B.
The Mugen S2000 is a bitch to drive fully loaded, but i'm just trying to get the best car in the class for the specific race.
There are a couple of nice speedway tracks, with only 3 bends, these are easy and net you between 5,000 and 24,000 per race, even my kids can do these, over and over again. Ch-ching.
The mugen civic is good, and the RS6 on the heavyweights, fully specced to around 650 KW is amazing, but corners quite poorly.
Just bought a 350Z track car, and fitted a supercharger (instead of the huge KW turbos) and seems more manageble for the Nissan Z car challenge.
Haven't done the classic cars yet, have you.