Tier 1 units are among the best candidates for an Armor team, and often provide unique and unparalleled advantages. These units should make up the core of your team as they are the most consistent and reliable options available.
Tier 2 units provide more specific roles in an Armor team and should be considered to cover any weaknesses. These heroes are among the best armor heroes in the game, surpassed only by heroes in Tier 1.
Tier 3 units have decent match-ups against most units, and should be used to fill any gaps in a core Armor team. Armor buffs and support may be more important in order to patch up any major weaknesses or enable certain roles.
Tier 4 units generally have awkward stat lines or other conditions that cripple their usefulness. Armor buffs greatly improve their usability, but they will still generally be outclassed by units in higher tiers.