It depends on what sort of driving I'm doing and the mood I'm in.
On the way to work I listen to a mix of everything (just chuck my iPhone on shuffle), which includes pop, rock, jazz, R&B etc.
When I'm cruising around slowly at night trying to pick up chicks or whatever I will have R&B playing. With some of the songs you can hear me coming down the road from about 200 metres away Gotta love the sub.
If I'm in a driving mood, it's gotta be trance. It gets me pumped and puts me in another mind set.
Had a good 300km drive tonight Was playing the 2008 Ibiza Trance Anthems CD. For trance fans, it's a great buy. Check out the album on iTunes. Best $16 ever spent.