Adamantite Ore

Your world rolls either this or Titanium, never both, and the choice is locked the moment the third altar goes down. Neither one is better in a way that should change your plans. Every extra altar you smash seeds less ore than the last, so there is a point where you are breaking altars for diminishing returns and inviting Wraiths for nothing. Break enough for a comfortable margin, then go mine.

It sits deep, around the level where lava starts pooling, and the ceiling of the Underworld is thick with it after a heavy altar run. Lava helps here: it throws enough light to reveal veins in the surrounding stone from a distance even without a Spelunker Potion. Explosives will not touch it, so bring a pickaxe that can actually cut it rather than trying to blast a shortcut.

The forge is the real prize. Both third tier ores unlock a furnace that smelts everything the rest of the game will throw at you, and until you have one, later ore is dead weight in your inventory. The gear from this tier is a stopgap you will replace within a couple of bosses. The forge is permanent.