My favorite music while driving is basically anything on my ZuneHD. It's about the only place I use the thing (it was amazing on campus during college) anymore, but it's worthwhile: 32GB of driving music, a "favorites" shuffle list over 300 songs long and so much more on tap.
I'll go from listening to a fair bit of the Avenged Sevenfold discography, to Coheed and Cambria (Neverender live discs, typically), Chevelle, Tech N9ne, Wiz Khalifa, Mars Volta, Tool, Dirty Heads, Slightly Stoopid, Queens of the Stoneage, Killswitch Engage, Thrice, Incubus, Kid Cudi, 311.
Pretty much everything I'll listen to I'll end up listening to while driving at some point.
Probably one of my favorite road songs, that's not from one of the artists above, is OAR's Road Outside Columbus (the live version of That Was A Crazy Game of Poker is usually played before or after that, for good measure

). It makes me nostalgic and sometimes a little sad, I miss Columbus so. Awesome song though.