Vertebra

Nearly every common enemy in the biome drops it, so the stack builds itself while you are down there doing something else. The reason to care is Battle Potions, which eat through the supply faster than casual play replaces it once you start farming events or chasing rare drops. A night on the crimson surface, or one pass through the caverns underneath, tops you back up.

The corruption equivalent covers most of the same recipes, and since the crafting change Unholy Arrows are no longer something only corruption worlds can make. Leather is the remaining gap: Rotten Chunk turns into it and this does not, so a crimson world has to find another way when a recipe asks for leather.