Rockdoor Backdoors from 3 different ranges, Ultimate, Elite and Select. Many doors in the ranges can be used as a front or rear door.
One door worth pointing out is the Rockdoor Vault. It comes without key access from the outside making it even more secure and practically impossible to break in from the outside when selected with laminated glass. How many times do you really lock the back door from the outside?
To be honest though most doors could be used as a backdoor but these are the most popular ones below