The race cars.
Simple, yes, the road cars are great and without the road cars I wouldn't have bought the game as I feel road cars in the earlier games are what made it, you'd buy your main car and zoink it up to 900BHP then go beat the world, but in GT4 the racing cars are so exciting.
Stepping from a C9, a 2J or a TS020 into a road car just feels... Gah, not as... enjoyable, sometimes I feel I'm being held up because the car that was originally designed for 400 or whatever horsepower is trying to handle almost double that (and in some cases more), I just feel the race cars suit the circuits more in Gran Turismo. Forza's gem was it was the opposite, not many people at all wanted to race the racing cars and if anything it was a, b and s class road cars that were the better, in GT I feel the race cars edge it.