Healing Potion

The number on the bottle is rarely the constraint. Potion sickness is. Every drink locks you out long enough that a boss fight realistically gives you two or three heals no matter how many you carry, and the skill is spending them before you are one hit from dead rather than after. Drinking at half health because you know a big attack is coming beats drinking on a sliver and getting clipped mid-animation.

The Philosopher's Stone and the Charm of Myths cut that lockout down. The accessory slot they cost is usually worth it for any fight you keep losing at the very end.

Before Hardmode you are mostly cracking Underworld pots for these, which is a good reason to smash every pot you pass on a hellstone run. Once Hardmode begins the Merchant simply stocks them, and from that point there is no excuse for walking into a fight short.