My judgement is somewhat hampered by not knowing how you pick each tier and what each tier is supposed to represent, but here goes:
The Desh plating is made of an inferior composition that's designed to be slightly lighter (making you faster), but also being a weaker protection. So I'd put it as Tier 1.
The Agrinium armor is designed for droids working in hazardous conditions and only requires the first level of upgrade feats. However, it does provide 50% immunity to heat, electricity, and energy. I'd mark it Tier 2 based on that.
The Quadranium armor is an advanced design of the Agrinium and the metal itself is used to make starship fuel tanks. I'd make it one tier above whatever you make Agrinium.
Diatum Plating is hands down Tier 3 armor. Allowing immunity to 75% of melee damage, this material is designated as "[p]rohibatively heavy" and is used to protect critical structures in asteroid fields.
The Capacitor Armor is the equivalent of the Diatum Plating, except that it protects against energy, ion, and electrical damage. It is Tier 3 as well. As a sidenote, this item is only available in-game at the HK Factory, in the storage room behind the elevator right after you enter (specifically, if you go into the factory and then turn around to face the elevator, it should be the storage room on your right).