Copperhead is a version of AOSP that focuses on delivering the most secure Android experience possible. I've been using it for a year now and have been quite happy with it.
Unlike other custom ROMs that strive to add lots of new functionality, Copperhead runs a pretty vanilla version of AOSP. Also, while the first thing you usually do when playing with a custom ROM is to add root access to the device, not only does Copperhead prevent that, it also requires that you have a device that has verified boot, so there's no unlocking the bootloader. This is to prevent malicious code from getting access to the handset.
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