[Tepix]

Depends on how open you want to be. Is CopperheadOS open enough for you? Then get a Nexus 5x/6P or a Pixel with the OS preinstalled.

[vedanta]

Yep, Lineage is available on a 5X, Samsung S7, LG G5, etc.. Libhybris/Halium uses the driver parts of AOSP if I understand correctly, and Lineage uses ART in addition, they are all dependent on AOSP in some way currently atleast.

Google's project treble might make haliums' job a bit easier, so you might see halium on more devices than you'd think. Just speculating.