Molten Helmet

Nothing available before the Wall of Flesh comes close to this on defense, and it adds swing speed, damage and critical rate on top, so the common plan for the last stretch of a world is to skip the middle sets entirely and go straight down for it. Players who are not playing melee still borrow it regularly, purely because the padding is that far ahead of the alternatives, even though they give up their own class bonuses to do so.

The gate is the Underworld run itself. Hellstone will not break without a Nightmare or Deathbringer Pickaxe, so the world evil boss has to go down first. Mining the ore releases the lava sitting under it, which is what actually kills people down there, and an Obsidian Skin Potion buys you long enough to strip a whole vein without thinking about it. Smelting happens at a Hellforge, and since those are already standing in the ruined houses, most players hammer one loose and haul it home rather than crafting while standing in Hell.

The fire protection is easy to undersell next to the defense number. With all three pieces on you cannot be set alight at all, which removes the burn that hellstone and meteorite hand out to anyone who touches them, along with the fire attacks that fill the Underworld. Mining either of those materials stops being something you plan around, so the second trip down is far calmer than the one you made to get the armor. It does not make you immune to lava itself, so the pools will still kill you. The flame trail behind you is decoration, and the helmet and greaves visibly pulse as you walk.

The first Hardmode ore set replaces it and the gap is not close, so there is no reason to cling to it once you are smelting new bars. Keep it in a chest even then. Nothing gives you the burn immunity again for a long while, and any building or mining you do below is much less irritating with the set on. Expert and Master change nothing about how any of it behaves.