From a game play standpoint, trillium/living steel is the most powerful. However, as the title suggests, what would be the most powerful lore wise?
By powerful, I mean durable/indestructible while also providing the user with any kind of power.
From my understanding, based on what ive from inside the game and online, there are a few candidates that qualify to be top tier:
- Thorium - This rare metal combines the weight of lead with the strength of steel. Orcs prize it for weapons because the extra weight allows a skilled user to strike with more force. Thorium armor is amazingly heavy and strong — only adamantine pierces it with any degree of reliability. Thorium is heavier than most metals. An item made from thorium weighs twice as much as the same item made from other metals
- Adamantite - In Warcraft lore, Adamantite is considered to be the next grade up in strength from Thorium.
- Saronite - The quest "Pure Evil" shows how strong and resistant the evil metal is. The Saronite ore collected was completely unscratched by the attacks from Crusade Commander Korfax's Arcanite Reaper. The ore was also completely resistant to the nature magic used against it by Rayne and Rimblat Earthshatter. Finally the ore was struck by the powers of the Holy Light by Avenger Metz, and the ore reacted but seemed to reflect the spell back at everyone in the area. The metal itself is very strong, while being 1/4 the weight of steel.
This would seem to be the main reason the Lich King chose to use this evil ore to create his fortresses in Icecrown as well as create his war machines, allowing them to be resistant to whatever attack was launched against them. It is likely that since the Lich King is so powerful, and his undead minions have no will of their own to speak of, they would be resistant to the evil of Yogg-Saron.
It also has powerful properties that are useful against undead, who's animate flesh is nigh indestructible. When forged into a weapon, not only would it tear the body apart but would obliterate the soul as well
- Titanium - Not much is known about this ore. I can only assume from the name and from its real life counterpart, that it should be very powerful.
- Elementium - Elementium is an extremely rare metal, found only in some of the most remote regions of Azeroth. It is believed by archaeologists to have been formed during the prehistoric reign of the Old Gods, in places where there were high concentrations of elemental creatures. In fact, elementium isn’t native to Azeroth, and is found only near high concentrations of elemental creatures because they bring it from the Elemental Plane where it is actually quite common. The unique properties of elementium allow it to absorb and channel elemental forces. Skilled smiths temper elementium in blistering forges, adding to the quality of the item. Elementium is so heavy that to craft armor from it would leave the wearer virtually immobile, so the metal is usually only used to craft weapons.
It was revealed that the Twilight's Hammer forged a new set of armor plates made of elementium for Deathwing.
- Obsidium - "Obsidium" could be attributed to "Obsidian" which is a glass-like material, and has been argued to be one of the sharpest known materials in the word. This assumption holds true since the artwork for "Obsidium" is shaped like flints, or shales..rather than the traditional Bar type ore.
- Pyrite - Though i am not sure of its durability, its destructive nature seem very strong. This is based solely on the Flame Leviathan encounter in Ulduar where one uses liquid pyrite to defeat the boss.
- Trillium - Almost nothing known about its strength.
-Living steel - Almost nothing known about its strength.
- Arcanite - This grayish metal looks dull even when tempered, but it holds a keener edge than any other metal on Azeroth. Arcanite is extremely flexible and can be worked into a springy armor that turns aside the mightiest blows. Only weapons, armor, and shields normally made of metal can be fashioned from arcanite; a warblade can be made of arcanite, but a tauren totem cannot.
- Dark Iron - Considering one needed to smelt and forge this inside of an instance, it would be wise to speculate that this metal held considerable capabilities.
- Frozen Rune - These were needed to craft the Icebane armor from old naxxramas. Not much else is known.
- Mithril - By some, Mithril is one of the toughest ores in Azeroth. Mithril is often mined by the dwarfs. Therefore the Mithril order was found.
- Truesilver - The magical and mysterious truesilver is greatly sought for its arcane properties, especially for its use in armor treatments. By the rigorous application of dweomers, armorers boast that this armor treatment "can give mortals a taste of godhood". While armorers are not immune to hyperbole, there’s little doubt this treatment produces extraordinary transformations. Only heavy armor may receive the truesilver treatment.
These are all the ones i can think of off the top of my head. So if theres some i missed post it and ill update it.
So, which is the most powerful? My money would be on saronite, but im probably wrong.
EDIT 1 : Added Truesilver and Mithril
EDIT 2 : Added more info about Obsidium. Thanks Stomperz