Obsidian

Obsidian appears wherever water and lava touch, so you never have to go looking for a deposit. The standard farm is a pool of water sitting above a pool of lava. Dig down through the crust into the lava, then cut a narrow side channel so water trickles back down onto it. Every block you mine lets the two meet again and a fresh one forms. It runs until one liquid drains away, and with liquid duplication it does not stop at all.

There is no obsidian bar, which surprises people. The ore feeds directly into Hellstone Bars, the Obsidian Skull and everything built on top of it, Obsidian armor, and a pile of furniture recipes. Nearly all of that demand lands inside one pre-Hardmode window, so a single good farming session covers a whole playthrough. It also survives lava, which makes it the sensible material for lining the walls of anything you plan to build near the Underworld. Leftovers can go through an Extractinator if you want sand without hauling it.