Iron Ore

A world generates with this or with Lead Ore and never both, and nothing you do changes which. Lead gear comes out slightly ahead on paper, but nobody stays in either set long enough for it to decide how a run goes.

The anvil is the real reason to gather any, and the merchant will sell you an Iron Anvil outright if you would rather spend coins than dig for it. Past that and a starter pickaxe, very little demands the stuff, and slimes and crates keep dripping small amounts into your inventory on their own. A dedicated hunt for it is usually wasted daylight.