At a minimun i would suggest one that has; a lcd screen so you can see what it is recording - a shock sensor so if your car gets jolted by a accident, lets say, the camera automatically records that last segment of video clip in a protected area on the sd card so it won't get erased when the camera loops recording - the ability to write over previous recordings, otherwise known as loop recording, at least 2 hour capability, you'll probably need to buy an 16gb sd card - some cameras have the ability, for more $, to record your speed and gps location - at least 1080 hd resolution with 140 degree viewing area - some cameras have the ability to beep at you when your stopped at a light and the car in front of you moves, to let you know to go, another option is it alerts you when you're driving and you sway out of your lane, it beeps - ability to record at night.
You can look on amazon at vinpie, papago and others. Read reviews and see which one fits what u are looking for.