Absolutely Broxigar.
The only mortal (only being of any kind?) to ever actually strike a wound against Sargeras. I think he may be one of the most awesome characters in the entire lore, and it's a shame he's not very well known.
History of Brox:
-Fights in all 3 wars. Survives them all. In the Third War, against the Burning Legion, he is the only member of his unit to survive. He is pissed about it. As we all know, Orcs have this thing about glorious death, and because he's too badass of a warrior to get killed, he feels somewhat slighted.
-Is pulled backwards through time to the War of the Ancients. Is now the only Orc that exists. Kills a lot of demons and as a result, is given a special enchanted wooden axe by Cenarius. (The axe needs to be a Legendary. See below.)
- Fights at the Sundering. In order to buy his friends time to do whatever they're doing, he leaps through the portal the elves have made, and proceeds to embarrass the Legion. He murders so many demons, they have to climb over their comrades' corpses to get to him. He continues to murder them, screaming and laughing the whole time. We're talking badass, front-line, kill-dozens-of-mortals-at-a-time demons. Succeeds in holding back the ENTIRE freaking Burning Legion from entering Azeroth.
- Is confronted by Sargeras. The most powerful evil being in the Universe. An immortal, invincible Titan, ruler of the Legion.
- Proceeds to run screaming at freaking SARGERAS and plant his axe in the Titan's leg.
- Is summarily killed by Sargeras with a gigantic flaming sword.

At this point I assume he went to Hell on purpose and killed the devil, then dragged the carcass back to Orc Heaven and used it as an unusually supple throw rug.

This dude needs to be in the game somehow. And the Axe might be the most awesome weapon ever. What other weapon has wounded a Titan?