I've been hunting for the ultimate ROM for my Nexus 6P and so far these are the ROMs that I've tested so far..
For some weird reason I can't seem to install Dirty Unicorns ROM which is highly praised.
Every ROM is unique in its own way but they share a lot of common features. For instance Cataclysm's smart radio is an excellent feature which could help you save a little battery life. Whereas there's no limit for Pure Nexus when it comes to customisation. But my favorite so far has been Chroma as it strikes the right balance between customisation, additional features and stock experience.