Obsidian Horseshoe

This is one of the few combines in the game that leaves you worse off than the parts. It pairs fall protection with the fire immunity you want for hellstone mining, which sounds right until you look at the other branch off the same skull. That branch keeps the burn immunity, adds knockback resistance and a defense bump, and stays a valid ingredient in later recipes all the way through hardmode. The horseshoe is a dead end and cannot be taken apart afterward.

Fall damage is also the single easiest effect in the game to cover some other way, since almost every movement accessory line handles it eventually. Build this only if you are genuinely short on accessory slots for one hellstone trip and have no interest in the shield line, which for most runs is not true.