Shackle

A single point of defense is close to nothing on paper and genuinely useful in the first hour. Before you have armor worth the name, a zombie hitting you for slightly less is real, and the slot it takes is one you probably have nothing else for. Blood moons hand these out in bulk, so getting a couple is a matter of surviving one night behind a wall.

Reforge cost scales with the item, and this is one of the cheapest things in the game to roll. A warding roll turns it into several times the accessory it started as, for less gold than a single attempt on something expensive. It is also a good way to learn how the tinkerer works, since you can hammer the button without going broke.

It stops mattering as soon as you have something that gives defense plus an immunity. The skull and shield options at roughly the same point carry burn or knockback protection along with the armor value, and the combined version carries both. Keep the shackles anyway. They feed a couple of crafting chains later, so a chest of them is worth more than the coins you would get for selling.