Spartan Armor

Mjolnir Mark IV
Default Armor
Mjolnir Mark V
Unlock the “UNSC Spartan” Achievement
CQB
Default Armor
EOD
EOD “Body”, beat “Tsavo Highway” on Legendary.
EOD “Shoulders”, beat “The Ark” on Legendary.
EOD “Helmet”, beat the Campaign on Legendary.
EVA
EVA “Body”, complete “Tsavo Highway” in Campaign on either Normal, Heroic, or Legendary.
EVA “Shoulders”, complete “The Ark” in Campaign on either Normal, Heroic, or Legendary.
EVA “Helmet”, complete the Campaign on either Normal, Heroic, or Legendary.
ODST
Unlock the "Spartan Graduate" Achievement
Rogue
Unlock the "Spartan Officer" Achievement
Scout
Scout “Body” Armor, unlock the “Too Close to the Sun” achievement.
Scout “Shoulders” Armor, unlock the “Mongoose Mowdown” achievement.
Scout “Helmet” Armor, unlock the “Used Car Salesman” achievement.
Security
Security "Helmet" Armor, unlock 1000/1000 gamescore.
Security "Shoulders" Armor, unlock 700-800/1000 gamescore. (glitched)
Hayabusa
Hayabusa "Body" Armor, unlock 5 skulls.
Hayabusa "Shoulders" Armor, unlock 9 skulls.
Hayabusa "Helmet" Armor, unlock all skulls.
Hayabusa (Katana)
Hayabusa "Katana" Armor, unlock 1000/1000 gamescore.
Recon
Not unlockable, only obtainable directly from Bungie.

Elite Armor

Combat
Default Armor
Assault
Default Armor
Flight
“Flight” Body, you will need to beat “Tsavo Highway” on either Heroic, or Legendary.
“Flight” Shoulders, you will need to beat “The Ark” on either Heroic, or Legendary.
“Flight” Helmet, you will need to beat Halo 3 or either Heroic or Legendary.
Ascetic
“Ascetic” Body, you need to unlock the “Up Close and Personal” achievement.
“Ascetic” Shoulders, you need to unlock the “Overkill” achievement.
“Ascetic” Helmet, you need to unlock the “Steppin’ Razor” achievement.
Commando
Unlocked by Bungie for everyone in 1.1 auto-update.