I don't know exactly what class of car I've spent the most time in but I generally prefer to drive anything in the B to R2 range.
I love the purpose built race cars like the DTMs and ALMS cars, and generally any fast GT car or touring car. In particular, I have a soft spot for the Ford GT, Mercedes SLS, Corvettes, Gallardo, Aston Martins,etc....
i did the career mode in mainly A & C class. A Lotus Exige in A class with a cracking Gilera replica paint job (tribute to Marco Simoncelli) and a Mk5 Golf GTI with a 4WD swop in manufacturer's Reflex Silver for class C.
Would have done more S class races but couldn't really find a motor to cut the mustard vs. knobhead Rossi in a Ferarri 599XX - that car has immense straight line speed and when you are fighting to come through the pack catching it can be nigh on impossible