The Lemans cars are all pretty quick, haven't tried many of them The R8 and R10 are quick but if the game has any bearing in reality the Pescarolo LMP1 cars should be faster.
The Mercedes-Benz Sauber Mercedes C9 Race Car '89 if it's anything like it was in GT4 will be ludicrously fast, with the right setup it could lap monaco faster than the Formula Gran Turismo and had a much higher top speed too.
The Jaguar XJR-9 LM Race Car '88 should be amongst the fastest too.