Well, it depends on what sort of humor you like, doesn't it? Do you like clean humor? Do you prefer your comedy blue? Would you rather hear stories, bits, observations or one liners? Who have you found funny in the past? If you can give me a basic idea of what you find funny, I'm sure I could recommend some comedy you would enjoy (since I basically like it all).
Offhand, I'd recommend Daniel Tosh. I just discovered him recently (he was mad when I planted that flag), and he's great. Demetri Martin is also fantastic, though a lot of his humor is visual, so I don't know how well it would work as audio (but definitely get the DVD "Person," which contains two Demetri Martin Comedy Central specials that are both fantastic).
Steven Wright is good for a road trip because you can start or stop his album anywhere and not feel like you're missing the flow of the show, since it's just an hour of one liners. On the flip side, story-based comedians like the Blue Collar guys (especially Ron White) can also be fun for a road trip, since listening to amusing stories while you drive down the road can make the time fly by.
George Carlin is my favorite comedian of all time. Just buy every CD he has put out and listen to them in chronological order (or, for a nice bi-polar touch, try alphabetically; listening to Class Clown and following it with Complaints and Grievances, you'd swear you were listening to two different people). You could drive across the country and not realize it cause you'd be laughing too hard to care where you were going.