yeah but i disabled moods so my dward must work his ass off. i just probably have to do a shit ton of copper stuff because we have no iron
The article you're looking for on the df wiki is this one:
http://df.magmawiki.com/index.php/MeltMelt gives you info on the yield of melting metal objects. As far as I know, there's no difference in experience gained by crafting any one type of object.
For armor, one armor breastplate returns 0.9 bars of metal. Once the using-multiple-bars-to-create-one-item system is in place, breastplates will be less effecient b/c they will require 3 bars to create. However, under the current system,
crafting and melting breastplates is the most efficient; it will only lose you 0.1 bars per craft (plus fuel).
For weapons, the best you can do is a 50% return by crafting trap weapons.
Enormous corkscrews, giant axe blades, and
menacing spikes all have equivalent 50% returns, so make whatever you think you might use the master work versions of - corkscrews if you're going to craft a bunch of pumps or spikes if you're using them for defense.
For blacksmithing, it looks like cages are the best possible return. Wiki indicates that a cage will return 1 bar of metal, giving you a 100% return of the metal. (Note: again, this only applies under the current bugged system of metal bar use. Some mods reinstate a multi-bar requirement so if you're playing a modded game, follow this advice at your own risk.) If you want to level a blacksmith, have him
craft cages on repeat; 100% of the metal will be retained and only the fuel will be lost. (Second note: I haven't tried this one, as the map I did level skills like this on, I had a *ton* of metal and no wood, so I had my blacksmith set to making lead bins and barrels on repeat. I've tested weapons and armor myself, but not cages.)
Remember that if you're melting items and reusing metal, your dwarves will be upset if you melt down their masterwork items. I reserved these items for sale to caravans or weapon traps to raise room values.