I like tapes because they don't skip or scratch. However at the moment I use CDs in the car, although I'd be better off whacking a big USB stick or SD card in the front of it, then it wouldn't skip or warp in heat! As soon as I work out how to make it work...

8gb memory card in my phone, bluetooth reciever plugged into the aux-in of my head unit.... sorted

Myself I go for MP3...

Easy option for me, I'd probably listen to normal cds too but MP3's would be more popular with me.