There are so many favorites, it's so hard to pick... That's why I prefer to always keep *all* of my music with me in the car (it's about 1600 CD's worth of it by now). My car player is the good old empeg, with 2 x 120G hard disks.
I had an iPod link in my Aero (and my LR) and agree, it is the only real option if you love music (and hate the drivel on the radio!).
Can't wait until more car makers add Media Card options to their stereos. Surely this, or a removable HD, has to be the way to go!