Caterham A3 class - On windy tracks without long straights the car is way too fast compared to others, balanced out by the fact that its top speed is really low, but you find if you're driving the caterham the AI doesnt use the full top speed potential of their cars, for example the Ferrari 612 will go well over 200mph, though the AI will not do even close if you're in a caterham.
Ferrari 458 A2 Class - This car thrashes all of the other cars in A2, performance wise it is about equal to the Mclaren Mp4-12c (which is class A1). There is no challenge in A2 if you use this car.
Bugatti Veyron A1 class - Stuart (main rival) drives this car and off the start he just flies off into the distance, acceleration wise the car has no competitor, however stuart drives it really slow in corners, so you can win the series in just about every car, i did it with the Mclaren Mp4-12c. The handling on the Veyron is not the best but due to its massive straight line speed advantage (in a series that unlike previous is all about high speed), using this car completely kills any challenge in A1.