Depends on my mood! If I'm on my own (or with my daughter) in the car, I listen to a range of styles of music from Celtic and mellow country to heavy rock .... I love singing along (I pick up the words very easily, and remember them - but singing-wise, I can't carry a tune in a bucket, so I only sing on my own or with Lady P who doesn't care!!).
If I really need to chill out, my go-to track is Albatross (Fleetwood Mac) - it transports me to somewhere calm and peaceful when I really need that time out.