There are too many best masks. Some of my favorites are the original six masks, the Toa Metru masks, Kakama Nuva, Akaku Nuva, Faxon, Tryna, Toa Phantoka masks, Hau Mistika, Pakari Mistika, Ignika, Avohkii, Gresh helmet, Ackar helmet, Strakk helmet, Hydraxon helmet, and a bunch of others
My least favorites include the 2001 Turaga masks, Hau Nuva, Calix, Elda, Zaath, Kaukau Mistika, Noble Jutlin, Noble Avsa, Skrall helmet, Malum helmet, Atakus helmet, and maybe a few more.
On a side note, was the Olmak inspired by the face of Avak prototype? they look very similar.