Hellstone Bar

Nearly every run funnels through this material. It is the last big gear jump before the Wall of Flesh, and more importantly the only pre-Hardmode pickaxe that will bite into Cobalt or Palladium is the Molten Pickaxe. Skip the trip and your first hours after the Wall are spent unable to mine the ore you just spread across the world.

That makes Hell a checkpoint rather than an optional detour. Go down with burn protection, some way of crossing lava, and enough time to bring back a real haul instead of a handful. You will want bars for the pickaxe, for armor, and usually for a weapon on top of both.

The bars will not burn up in lava, which matters when the vein you are chipping at is half submerged in it. Usefulness does not stop at the Wall either. The same bars feed the Imp Staff, the bug net that survives lava, and much later the Drill Containment Unit, so do not sell the leftovers the moment Hardmode starts.