That mostly depends on your prefered class and shaders. For instance, masks with hoods (Eris, Rahool) would look much better on cloaked Hunters than on bulky Titans and cowl-less Warlocks so you could rule them out if you're not mostly a Hunter. Engrams are always nice looking when they match the equipped shader, but you might feel that you have to keep the same blue/purple/green shader on at all time just so it fits, especially if it's a color you dislike. EDIT: I saw the options you said you had (Atheon, Exotic, Xür, Skolas) and I'd use the glue on the Exotic mask. It's more noticeable than Xür and it's much easier to make it fit with your current gear than Skolas and Atheon.