Feather

Harpies drop these constantly once you are building up in the sky, and it is easy to treat them as junk for a whole playthrough. Then hardmode starts, you want your first pair of wings, and you need a pile of them at once. Keep a stack in a chest from the moment you meet your first harpy.

Harpies spawn in the upper part of the world, so the fastest way to gather is to build a small platform high above your base and stand on it. That doubles as a floating island route and gives you somewhere to fight them without falling.

Featherfall and Gravitation potions both want them, and gravitation potions in particular are how a lot of people find floating islands in the first place. Newer versions also let you turn them into Harpy Blocks for building.