Continuing the discussion from What are the 3 most powerful kanohii masks?:
So, here's what I'm thinking.
Rankings for great masks are twofold: Unique and Crafted
Uniques, so far as we know, are masks which cannot or have not been made by any MUI. (IE made by the great beings, or some other unknown force), or there are not any known duplicates in one universe. The Uniques are more powerful than Crafted, but do not control any of the fundamental MUI forces, so the three legendary masks are technically Unique, but do not fall into that category because they control either creation, time, or life. The known Uniques are: the Avohkii, the Kraahkan, the Great Mask of Light and Shadow, the Olisi, the Rua, the Aki, the Olmak, the Rode, Great Mask of Elemental Energy, Great Mask of Psychometry. There may be others, so keep me updated.
Crafted Masks are masks that are able to be made by matoran by any process. The only loophole is the Vahi, which was indeed crafted by Vakama, but as it controls a fundamental MUI force, (time) it is a legendary mask. All crafted masks have at least one duplicate and are less powerful than most masks. (noble and powerless worn respectively by turaga and matoran) There are too many known Crafted masks to be listed here. See BS01 for a complete list of all referenced kanohi in the BIONICLE story.
There are other masks that are neither Unique or Crafted, the Nuva masks. (Eg. the Nuva masks worn by the Toa Nuva.) They are more powerful than their traditional Crafted variants, but they are also transformed by energized protodermis. I think it would be best if we should simply call these Nuva Masks.
Please, if there are any more exceptions, or any other Transformed/Unique masks tell me and I will edit the topic. =)