I had multiple favourites in no particular order:
1) Mark VI
Always looked nice.
2) Hayabusa (Katana)
Symbolised you were capable of earning at least 1000gs in the game. Used to be for just the original 1000gs. Implies the wearer has put the time into the game and should be a decent opponent.
The be-all to end-all armour. Symbolised a player had earned Bungie's respect. Later showed a player was capable of earning the difficult Vidmaster achievements from 3 and ODST, which required significant time and investment by the player and his/her friends. This armour lost some of its appeal to me when it was handed out for free in waves to the community, de-valuing the effort of those of us who earned it through one of the two methods before.
4) EVA (helmet)
For months in magazines, we the community thought it was a modified version of the SPARTAN III SPI armour before being revealed to be the EVA MJOLNIR helmet. Still the closest thing we had to in-game S-III armour.
After writing all that, I hope Halo: Infinite has multiple variants of armour that are difficult to obtain, not just the generic beat the game on Legendary and REQ pack RNG. Would be nice if the armour meant something.