Hellforge

Every forge of this kind in the game is either standing in a ruined house in the Underworld or waiting inside a crate. That makes the first trip down a collection run as much as an exploration one, and how many you bring home depends on how much digging you are willing to do.

Pulling one out of the Underworld needs a pickaxe you can realistically only own after beating the world evil boss, and explosives will not do the job for you before Hardmode. The intended path is to smelt your Hellstone at one you found in place, then come back for the furniture later. Once it is out of the Underworld the rules relax and any pickaxe will move it again.

Beyond Hellstone, it is the base of the Hardmode forge that handles the top tier ores. Take at least one, put it somewhere convenient in your crafting room, and you have spared yourself a return trip at exactly the point in a run when you least want one.

It does everything an ordinary furnace does and lights the room while doing it, but it will not count as the light source for a house, and the mid-tier Hardmode metals do not need it at all. Those still smelt at a plain furnace.