I've been having fun with the Audi R8 Race Car '01 and Peugot 905 Race Car '92. Both have tons of grip and provide easy wins even below the 650pp limit against any opponent car.
I used the Zonda LM race car detuned of course to meet requirements ! I've also used a RM Corvette ZR1 detuned to meet requirements and the Mazda Furai detuned to 650pp with 425hp but don't be fooled by tge hp because this car kicks arse !