Unicorn Horn

Unicorns charge in a straight line and hit hard for the point in Hardmode where you first meet them, so farming horns means picking a flat stretch of Hallow and letting them run at you rather than chasing them over broken ground.

The horns are worth far more in the crafting menu than at a merchant. Super Mana Potions alone justify keeping a stack if you play magic, and the rest of what they feed into is hard to get any other way. Selling them is the one thing not to do.