Gel

Torches. You will place thousands of them across a world and every one costs gel, which is how a material dropped by the weakest enemy in the game stays in demand right through to the last boss. It is also why the opening hour of a run is spent hunting slimes instead of exploring.

Every sticky version of a thrown item runs on it. Sticky Bombs, Sticky Dynamite and Sticky Glowsticks are the ordinary item plus gel, and the glowstick in particular is what you throw down a shaft whose bottom you cannot see. It builds the Slime Crown, Slime Blocks and Asphalt, and the list has kept growing, Molotov Cocktails included. It also loads straight into an ammo slot for the Flamethrower and the Elf Melter, which is the one use that actually drains a stockpile. Despite the tooltip you cannot eat it, and the only consumable it appears in is the Lesser Healing Potion.

Different slimes drop differently colored gel, and a slime that was recolored by paint passes its new color on as well. Merged stacks take the color of whichever pile you dropped onto the other, regardless of which was bigger, so one stray green gel added the wrong way repaints a full stack. Drop it on the ground, put it in an item frame, or leave the world and come back and it reverts to blue. None of this affects crafting, so treat it as display material for a collection room and nothing more.

A Slime Statue on a timer, with lava, a trap or a parked minion underneath, produces gel indefinitely without any attention from you. That is the standard build once wiring is available. Umbrella Slimes are the one source that occasionally comes up empty, though not often enough to make them worth avoiding.

Raw gel sells for almost nothing. Run it through a Solidifier with a little wood and it becomes Slime Candelabras, which sell for a genuinely useful amount per thousand gel spent. For an early character who already has an automated slime farm running, that is one of the better returns on effort in the game.