The R18 is a bit overkill for most of the races. It is ok for the LMP races, not good at night though.
I used the Ford GT 06 for the Willow race, fun race not to hard with that car and some minor tuning.
I used the CLK Touring car for the 24 Minutes of Nurburgring, easy win, did not need to pit.
Audi R8 stock with RS in the LMP races
Can't remember what I used in the others