Ancient Hallowed Headgear

Mechanically there is nothing to choose between this and the standard Hallowed set. Same protection, same bonuses, same four headpieces, one per class. What differs is the sprite work, which is the older look for the armor, and the station: the bars go to a demon or crimson altar instead of an anvil, which almost nothing else in Hardmode does. The game treats both families as one set, so a helmet from one and a chestplate from the other still switch the bonus on.

The Mask, Helmet and Headgear cover melee, ranged and magic, and all three add the same effect to their class bonus: land a hit and nothing can touch you for a moment. That rewards staying on the attack through the mechanical bosses, since backing off to heal wastes the window you earned. The Hood is the summoner option and trades class damage for minion capacity. It is the one piece with no older original, since summoners had no Hallowed headgear back then.

This covers the stretch between the mechanical bosses and Plantera. Once Plantera is down, the class sets that follow are a clear step up and the brief invulnerability no longer covers for thinner protection.