Mortar

Taken all the way down to base materials, one Mortar is 2 crafting steps deep and needs 1 intermediate — Distilled Water ×1 — which resolves to 3 distinct raw materials totalling 4 units, plus 2 crystals.

That bill outweighs 3,935 of the 13,602 recipes tracked here and 746 of the 1,591 Alchemist ones, and 4,464 recipes nest deeper.

Only 1 of those crystals go into the final step; the other 1 are burned by the intermediates, so the real crystal cost is 2 Water Shard.

Every step is Alchemist work, from a Lv 1 intermediate up to the Lv 20 finish, so one class covers the whole chain.

All 3 raw materials are gathered, and not one of their spots is timed, so nothing in the tree waits on the Eorzean clock.

Mortar is itself consumed by 140 further recipes, spanning six of the eight crafting classes.