Dirt Block

It is the block everybody has before they have anything else. The entire surface cave system is made of it and about half the underground is too, so a shelter on night one costs you nothing but swing time. It is also among the softest material in the game, one hit from almost any pickaxe, though grass growing on a block buys it an extra swing. If a build runs you short, mud decrafts back into it in Shimmer.

Grass spreads from block to block across dirt, and that includes Corrupt and Crimson grass. A base built out of plain dirt is a paved road for the evil biome to walk down the moment Hardmode arrives, and the same goes for a dirt floored farm or a dirt lined tunnel running past your town. Swapping the dirt around anything you care about for stone brick, wood, or any other material that cannot convert takes an afternoon and saves a much larger cleanup later. Do it before you break an altar, not after you notice the purple.

Grass applies to the whole block rather than to one face, so once it takes hold it shows on every exposed side at once. Wall the block in on four sides and the grass survives as markings on the surface. Bury it completely, all eight neighbors filled, and it gives up and reverts to plain dirt. That is the free way to strip grass off a block without replacing it. A Dirt Rod will also lift a block and set it down somewhere else without mining it, which is useful for shaping terrain you would rather not break. And The Dirtiest Block is a pet item that looks identical to the ordinary one, so keep it out of your building stacks.