These are the things you want
* Do you want to have a dashcam with a GPS receiver?
* Yes, it can confirm the location of the accident that happened.
* Do you want a dashcam which records speed?
* Units with GPS inthem normally record speed
* What kind of resolution do you want?
* 1920x1080 @30fps and 11mbps bitrate
* Do you want to have a little video screen?
* No, they waste power, are more expensive and are distracting
* Do you want parking mode?
* Only get it if it can be hardwired in with a device than can shutdown if the cars battery level gets too low
* With 1 or 2 lenses in the camera
* 1 is good, 2 is better, with 2 one faces forward, the other faces you, so if a cop said you were using your phone when you weren't you have proof
* Do you want a 3-axis acceleration sensor?
* Most things that are called dashcams have these as they use these to record the "event" and some lock the event file so it cant be deleted.
You will want a 32GB or larger Class 10 Micro SD card.
You also do not want to broadcast that you have a dashcam which means dont get one that stands out, get one that blends in with the car or can be hidden and dont tell anyone if you get into a crash, if they lie to their insurer and your video says otherwise, let them deal with it.