If you are looking for the best car DVR system then that system should have G-sensor footage look technology. If you are in an accident and you are unable to stop your dashboard camera to overwrite the accidental footage or you may forget to stop your camera then G-sensor will automatically stop camera to overwrite the previous footage of the critical event as it automatically detect that something went wrong.
Your car DVR should have a large capacity SD card. These cards are available in 16, 32, and in 64 gigabyte range at very low rates. Even you can have an external hard drive to store data up to 1 terabyte or more. This extra storage capacity is very important in case of a critical event. For more info, browse this site.
Camera of your car DVR system should be of high resolution. You can have clearer footage with finer details that may be blurred out in lower resolution videos like hand signal, number plate and turn signal will not be cleared in low resolution video.
Last feature which a car DVR should have is temperature tolerance level. You should keep a sharp eye on what temperature your car DVR system can tolerate before investing into any car DVR system.