That depends, do you really want best? Or best thats reasonable? Or cheapest....
Here's what I'd do optimally (and most likely will.) I'd bury a gopro hd hero3 in the tailgate somewhere. Run hdmi from it to a proper monitor. Run usb from it as well so you can do data transfers and it gets powered. The hero3's are now $200 and are wifi enabled. The app for iphone/ipad already exists, however it doesn't yet for android. There's even some work being done right now in the user community on reverse engineering the HD Hero backpack connector on the back so you could diy a trigger system for turning on and off recording, snapping 5MP photos, etc. These will also do time-lapse at a variety of intervals which could be useful.
You can go the LCD HU route and plug it in (not sure if any LCD HU's currently have hdmi in though, so you'd be wasting resolution by converting it) or make a custom mount in the dash for it. At this point you're halfway to having a full on carpc if you desire that too. I may be going the route of just fabricating an iMac
into the dash. This way I'll have the whole works right there with a very nice touchscreen. From there the possibilities are endless.