Variety. I am a "channel surfer" when it comes to the radio. Skimming through the stations - popular rock, heavy metal, classical, jazz, country, bluegrass... whatever sounds interesting at the moment.
Also listen to talk stations for a break from music. NPR sometimes, sometimes AM politics... then back to music. NPR sometimes is GREAT when they are talking about something non-political. Get tired of politics, though.
Every once in awhile I throw in Beatles... the range of their music is amazing. Their last albums sound completely different than their early albums.
Sometimes I'll swing into the local library for their "books on tape" (CD now). I have found that non-fiction is best for me - I don't have to follow a plot or anything while driving. Some of the "Great Courses" talks are really good.