It depends on the kind of driving I'm doing. For road trips lasting about 2 hours, I have a list on my ipod of various music that moves well and is just plain good. For normal commuting, I let my iPod shuffle away and pleasantly surprise me.
But for specifics, I enjoy cruising to music from the same period from my '79 Trans Am, especially rock instrumentals.
For a good hooning session or joy ride, something along the lines of hard rock or metal does the trick.
Off-roading . . . ABBA, don't ask why, lol
night driving - silly techno-ey music, sort of a guilty pleasure
cruising in the fox body - Joe Satriani stuff
cruising in the Oldsmobile - classical music or 20's jazz
anytime - can't go wrong with Weezer and Dwight Yoakam