What is the rarest Armor/Helmet in Halo 5?

Games Halo 5: Guardians
1

I think it is the HCS Athlon armor and helmet. What do you guys think?

2

I would say Achilles.

3

Recruit Charred

4

Either Achilles or HelioSkrill

5

I would say Achilles helmet. Many companies have the armor now.