Pixie Dust

Pixies drop it every time you kill one, and the hallow at night throws pixies at you nonstop, so the stack grows faster than any recipe consumes it. The best sink is Greater Healing Potions: the finished potion sells for well over the sum of its parts, mostly because of the Crystal Shards in it, and you want a full stack of them for boss fights anyway. An Alchemy Table makes each batch go further.

Fairy Wings also turn a profit, but other wings sell for more from the same Souls of Flight, so the dust is not the constraint there. Sell the dust or brew with it and put the souls into something better.