My fav's have been

Dunmer Indoril armor (Morrowind) or maybe Her Hand's (Tribunal).
Cyrodiil / Imperial Mithril armor (Daggerfall & Oblivion).
Imperial Studded leather (Morrowind & Skyrim)
Nordic Dragonbone (Skyrim), the Nordic Ebony plate armor (Skyrim) or Nordic Mail (Bloodmoon).

Dreugh armor for an Argonian (like in Morrowind - but this time with boots).

I know it isn't armor, but I would love to have Tower Shields return.

Adamantium armor, morrowind style dwarven armor, Dreugh armor (I love them in general).

I think what armor appears should depend on the setting though. However, every game going forward should include the Ebony mail and the masque of Clavicus Vile xD