I don't think I could narrow it down to one favorite song, but definitely I have two favorite albums that I like to listen and sing to while on road trips - The Eagles Greatest Hits, and the soundtrack from Wicked. For those of you who watch New Girl, remember that scene where Winston was in the car driving to Mexico to see his girlfriend and really jamming to Wicked? That's totally me.
When it's just short trips across town, like going to and from work, I like to listen to classic rock and/or 80's music in general. Right now, I've got a three month subscription to Sirius (it came with the car I'm leasing) and I've also been known to occasionally flip over to the 90's music channel and sing along with some jams as well, but that's pretty much a crapshoot - some of the music in the 90's was reasonably good, but there were some really terrible music phases at that point, too (though with the introduction of parachute pants, does that really surprise you?).
So what about you? Are you an awesome singer with a penchant for modern pop, or a so-so singer with a yen for Broadway musicals?