I hate to say it...but my stereo.....
Having GPS, back-up camera, DVD and iPod control in one 7" touch-screen unit is a godsend both on and off road.
I think my stereo would be a close second, I love being able to rock out to good, clean, and loud tunes on and off road. Plus my latest deck has an iPod/iPhone interface and bluetooth, it's almost too luxurious on road
And not that I should be doing this- But I found out that with the iPod interface I can put movies on my iPhone watch them when I drive long distances
The sound comes through the stereo and my phone rests perfectly on the dash for me to view. Again I know I shouldn't, but it's freakin sweet being able to do so!