Cobalt Leggings

Cobalt is usually the first thing you make once the Wall of Flesh is down and the new ore starts turning up underground. The chest and leg pieces are the same for everyone, so the only real decision is the head slot, and there are three of them. The Cobalt Helmet speeds up melee swings, the Cobalt Mask gives ranged builds a decent chance to fire a round without consuming it, and the Cobalt Hat cuts what your spells pull out of the mana pool. You only need one of them for the set effect, so build for the class you actually play rather than collecting all three.

None of the three versions is a dramatic jump over the best pre-Hardmode gear, but each edges past its predecessor. Melee players get a bit more damage and faster swings than Molten armor gave them. Ranged players give up a slice of damage for a much better critical rate than Necro. Mages come out ahead of Jungle on both damage and crit, paying for it with some maximum mana and a barely noticeable rise in what each cast draws. The defense is the part you feel first, since Hardmode enemies hit far harder than anything you fought the week before.

Your world picked one of the two first-tier Hardmode ores when you broke your first altar, so you get this set or Palladium, never both. Bringing the other one in means visiting a second world. There is no reason to bother for power alone, since whichever ore you were given carries you to exactly the same place.

Not long. Smashing more altars seeds the next tier, and Mythril or Orichalcum will be within reach well before you are ready for the mechanical bosses, with Adamantite or Titanium after that. Treat this as the set that gets you through your first Hardmode nights alive rather than one to invest in. Wearing the full thing leaves a faint string of afterimages behind you as you move, which is decoration and nothing more.