Older player here prefer D &C class venture into B once in a while.Prefer the old hotrods as I like to paint them in nascar themes as well as the drag cars.Also like the 70’s Trans Am series the 5 liter and the 2 liter classes. Ihave the new mototorsport but it really is lacking in the car and track dept, and it’s career mode sucks the will to play.
It’s broken because Miata
Amen to that brother!
The Miata is too OP in E,D,C and sometime even B class, minus Oval tracks or very fast tracks like Road America full. In general there are some cars who are way too powerfull and op in all division. T10 should just ban those cars from multiplayer. Begin a bit better is a matter, begin a powerhouse and meta game is another.
In a nutshell: D but to a lesser extent C are broken due to the Miatas.
If you really want to win and aren’t a Forza alien, you stand the best chance in a Miata as you can bung race tyres on and that’s all you need lol. I race in everything from E through S as for me they are the fun classes. D through A are my favorite but D and C are won most often by Miata drivers. Easy to tune by just plonking Race tyres on lol 
End of the day race what you want to chap
I have found that most of the muscle cars with an engine swap (4.3 inline 6) actually end up pretty competitive in B and C class (C class requires running slightly heavier). I generally run B class and slower, because it makes some of the games controller integration issues slightly less obvious.