Use cars that are in the same class as the opponents in each race. My recommendations:
Schwarzwald League B: Audi R8 4.2 or 5.2, Audi RS 6 Avant, BMW M3 Coupe '07, BMW M5 '08, Mercedes Benz C63 AMG, Mercedes Benz SL 55 AMG, Mercedes Benz SLS AMG or Mercedes Benz SLR McLaren (19 inch wheel option) '09. All with no or minimal tuning. For B-Spec, I'd just use an Audi R8 LMS Race Car '09, Audi R10 TDI Race Car '06 or better yet, an '05 or '01 Audi R8 Race Car. Other great choices for B-Spec include the Sauber Mercedes C9 Race Car '89 and AMG Mercedes CLK-LM Race Car '98.
Like The Wind: Any LMP1 or powerful JSPC race car. You might want to give Bob a Formula Gran Turismo or better yet, a Red Bull X2010.
NASCAR Series: No real choice here. Any NASCAR. Boring event.
European Championship: European car with about 500 PP I guess. Give Bob any European car you want, look at the ones I listed for B-Spec Schwarzwald.
DTC Championship: Any touring car, Lexus IS F Racing Concept.
Super GT: Any Super GT500 car. But give Bob a Formula Gran Turismo or Red Bull X2010. If you don't have these then give him a MINOLTA, Nissan R390 GT1 (road or race car), Nissan R89C Race Car '89 or Nissan R92CP Race Car '92.
American Championship: Cadillac CIEN Concept, Chevrolet Corvette ZR-1 or Z06 (C6), tuned Camaro, tuned Challenger, any Viper, any Chaparral, any Ford GT. Give bob a Chaparral 2J or one of the Ford GT LM cars, or maybe a fully tuned Z06 RM or ZR-1 RM.
Formula GT Championship: Only option is a Formula Gran Turismo.
Dream Car Championship: Super GT500 cars fit in, or less powerful LM cars. The GT by Citroen Race Car is a nice choice as well. But for B-Spec, use a Red Bull X2010 or if you don't have one use a Formula Gran Turismo or LMP1/JSPC car.