I'm on an RX-7 Infini-III for my next game, seeing how many different races I could win with it. Quite the firestarter, I must admit. And rallying with it isn't half bad:
All the Sunday, Clubman (I know, big deal), FR races, GT300 series, Grand Touring, Pure Sports, Smokey Mountain North, SM South, and Green Forest rallys. And those are races where the RX-7 isn't really out of place (racing it against wagons is fun the first time around, but after 3 other games, you should be able to follow the spirit of the rules).
So the Inifini must be king of 1st-day cars.