Guide:Armor progression

Progression in Terraria is marked by the player gaining access to new weapons, accessories, and armors. While weapons are the main determinant of combat effectiveness, armor also plays an important role by mitigating damage taken and passively boosting damage output. Choosing the ideal armor sets to use at each stage of the game is sometimes straightforward, but not always.

Earlygame
At this point, armor serves little purpose other than to boost your defense. You may obtain up to 20 defense with metal armors such as Platinum armor, but the resources required are better spent elsewhere. Therefore, it's recommended to simply wear Cactus armor or other basic, easy-to-craft armors until you can get your hands on a real armor set; whatever you wear at the start of the game will be replaced very quickly.

All classes
Mining Helmet / Gi / Obsidian Pants --> Practical set for exploring. Use Bombs for Obsidian.

Pumpkin armor (during Halloween) --> 10% damage boost; might increase your damage by 1 or 2 early on.

Magic
Wizard Hat / Gypsy Robe / Obsidian Pants --> Requires some luck from Traveling Merchant. Use a Diamond Robe otherwise.

Late pre-Hardmode
Sets that can be acquired after smashing at least one Shadow Orb/Crimson Heart, defeating a boss, and/or exploring environments more hostile than the Cavern layer.

Melee
Crimson armor --> Use to regenerate health while exploring.

Molten armor --> Beefy defense stat with good melee damage for its set bonus.

Ranged
Crimson armor --> Use to regenerate health while exploring.

Necro armor --> Comes a bit late, but is the only ranged-specific armor set in pre-Hardmode.

Necro Helmet / Gi / Necro Greaves --> Slightly more damage output, and only gives up ammo reduction.

Throwing
Ninja armor --> Easily destroys the Wall of Flesh with Beenades.

Fossil armor --> Stronger than Ninja armor but requires considerably more effort to obtain.

Magic
Wizard Hat / Gypsy Robe / Meteor Leggings --> Highest magic damage output.

Wizard Hat / Diamond Robe / Jungle Pants --> More focused on mana economy, but unnecessary after reaching 200 mana.

Meteor armor --> Ideal if you want to use the Space Gun for everything.

Crimson armor --> Classless set that mages can use for survivability.

Summoner
Bee armor --> Your only option as a summoner.

Early Hardmode
Forbidden Treads make for an excellent filler item in the leg slot, especially for sets like Adamantite armor that lack a useful set bonus. Their +2 bonus to minion capacity will usually outshine the paltry offensive bonuses of other leg slot items.

Melee
Palladium armor

Orichalcum armor

Titanium armor

Adamantite Helmet / Frost Breastplate / Forbidden Treads --> Melee speed set bonus is far more situational than +2 minion capacity. Many weapons aren't improved by melee speed bonuses.

Ranged
Palladium armor

Orichalcum armor

Titanium armor

Adamantite Mask / Frost Breastplate / Forbidden Treads --> Ammo reduction is a useless set bonus, so you may as well hybridize.

Magic
Palladium armor

Orichalcum armor

Titanium armor

Adamantite Headgear / Gypsy Robe / Forbidden Treads --> Awful defense, but has the best offensive stats for a mage who is open to using summons.

Summoner
Spider armor --> Use for summoning sessions, and keep wearing it because you have nothing better at this point.

Forbidden armor --> Poor stats, but an excellent set bonus for crowd control.

Pre-Plantera
Apart from Hallowed Bars, you now have access to the second tier of the Old One's Army, the rewards of which provide welcome upgrades to every class except magic.

Melee
Titanium armor

Hallowed armor --> Significant melee speed bonuses that can be useful with the Terra Blade and Death Sickle.

Chlorophyte Mask / Monk's Shirt / Squire's Greaves --> Highest melee bonuses that do not rely on set effects.

Ranged
Titanium armor

Hallowed Helmet / Huntress's Jerkin / Forbidden Treads --> Provides more minions and better raw stats than Hallowed and Chlorophyte sets.

Magic
Titanium armor

Chlorophyte armor --> Has alright defense for a mage set, but mediocre damage.

Hallowed Headgear / Gypsy Robe / Forbidden Treads --> If you've resigned yourself to the fact that you will never have any defense as a mage, use this set for maximum damage output.

Summoner
Squire armor --> Decimates crowds of enemies when used with Ballistas.

Squire's Great Helm / Spider Breastplate / Forbidden Treads --> An attempt at a tanky minion-based set. Use Spider armor for summoning sessions.

Pre-Betsy
While Plantera and Golem provide separate advancements to game progression when defeated, they are treated as one milestone by this guide due to their similar difficulty level. At this point you would be expected to have defeated Duke Fishron, Martian Madness, and the Pumpkin Moon. The Xeno Staff in particular provides a noticeable power boost to those who were using Forbidden Treads.

Melee
Chlorophyte Mask / Monk's Shirt / Squire's Greaves --> Highest melee bonuses that do not rely on set effects.

Beetle armor --> Potentially both the best offensive and best defensive sets, but skill-dependent and arguably overspecialized.

Ranged
Shroomite Mask OR Shroomite Headgear / Shroomite Breastplate / Forbidden Treads --> Highest DPS available when using the Xeno Staff. Doesn't require you to stay still.

Magic
Titanium armor --> Listed for lack of any better defensive effects for mages.

Spectre armor --> Due to Apprentice armor's poor bonuses, the Spectre Mask is the best offensive pure mage set at this time. The Spectre Hood will remain useful for a longer time, however.

Hallowed Headgear / Spectre Robe / Forbidden Treads --> One UFO minion is approximately as strong as the offensive Spectre set bonus, so for those who prefer to use minions, this set is viable.

Summoner
Spooky armor --> Use for summoning sessions.

Tiki armor --> Switch from Spooky armor for one extra minion.

Squire's Great Helm / Tiki Shirt / Forbidden Treads --> Switch from Spooky armor for more survivability.

Pre-Lunar Events
With the Old One's Army vanquished and a good supply of Defender Medals in your pocket, you can now purchase the best armor available before challenging the Lunar Events. The most notable of these armor pieces is the Valhalla Knight's Breastplate, which drastically increases the survivability of all classes.

Melee
Chlorophyte Mask / Valhalla Knight's Breastplate / Valhalla Knight's Greaves --> The best melee set in the game for overall offense and defense.

Ranged
Shroomite Mask OR Shroomite Headgear / Shroomite Breastplate / Forbidden Treads --> Highest DPS available when using the Xeno Staff.

Shroomite Mask OR Shroomite Headgear / Valhalla Knight's Breastplate / Forbidden Treads --> Barely qualifies as a ranged set, but has survivability.

Magic
Hallowed Headgear / Valhalla Knight's Breastplate / Dark Artist's Leggings --> Offensively superior to the Spectre Mask, and has constant regeneration.

Spectre Hood / Spectre Robe / Spectre Pants --> A fallback option for rapidly healing yourself and your teammates.

Summoner
Spooky Helmet / Valhalla Knight's Breastplate / Forbidden Treads --> A regenerating, minion-based set. Summon minions while wearing Spooky armor.

Shinobi Infiltrator's Helmet / Valhalla Knight's Breastplate / Forbidden Treads --> More sentry-based than the above set. Use with Frost Hydra and Queen Spider.

Endgame
All Luminite-tier armors except for Solar Flare armor are the best offensive armors for their respective classes. However, the Valhalla Knight's Breastplate can still be worn by ranged users and summoners, who do not receive any defensive benefits from their respective endgame armors.

Melee
Chlorophyte Mask / Valhalla Knight's Breastplate / Valhalla Knight's Greaves --> Remains defensively superior to Solar Flare armor against any attack with less than 172 / base damage.

Ranged
Vortex armor --> Immense DPS when stealthed, but only reaches full potential in multiplayer.

Shroomite Mask OR Shroomite Headgear / Valhalla Knight's Breastplate / Vortex Leggings --> Singleplayer set that emphasizes survivability.

Magic
Nebula armor --> Arguably the best armor set in the game, providing immense damage and regeneration bonuses.

Spectre Hood / Spectre Robe / Spectre Pants --> Mildly useful for pinch healing, but mostly outclassed.

Summoner
Stardust Helmet / Valhalla Knight's Breastplate / Stardust Leggings --> In contrast to the full Stardust set, this set provides survivability.