We’re strictly talking appearance here. I’m sure that mask powers have had plenty of discussion already.
To begin, I prefer the kind of mask where the mouth calls little attention to itself, or is overshadowed by other features. As iconic as it is, Tahu’s Mask is not my favorite. The second feature I like best is when there’s a middle ground of complexity to the geometry. With that said, three masks that would fit this bill would be those of Pohatu Mata, Onewa Metru, and Jaller Mahri. I’m not a fan of any of the Nuva masks.