In no particular order...
Blasting around El Capitan in one of my RX-7s. Apart from Nordschleife (obviously), its my favourite track, and I seem to beat the AI more easily here than any other place. The cars sound, accelerate and corner very well.
The Ruf RGT and Yellow Bird are also great drives, and a lot more fun than in GT3. In stock condition (they don't really need much tuning) theres almost no understeer, or dangerous reverse-travel oversteer for that matter. For some reason I can't make the M3s reach the same handling levels.
A classic, lightweight, medium-powered coupe is never wrong either. The Giulia Sprint GTA was my first drive in GT4, and I still take it for a ride once in a while.
Recently I've been driving the Peugeot 106 Rallye a lot. It's very light, and only needs as much power as the front wheels can handle. It handles well, and with racing exhaust it does sound like a little lion
It looks flash too, the only car I know that actually looks best in white. With three-spoke wheels, bodykit and lowered to the max its hardly your average grocery getter, and all those Skylines and Civics better get out of the way
And the Autumn Ring Mini is a great track. Even a slow car can be entertaining here, and it's great for drifting around too (fond memories from GT1, and the Impreza rally car)