In my experience, thus far the most damaging weapon I have seen is a mace, the great maul. However, my barbarian is only in the end of act 1 nm. What I would suggest doing, is getting a plain great maul and hanging onto it. When you complete Charsi's quest on nm difficulty, have her imbue the great maul, it will rock. The closest thing to a great maul is a pike, which deals 14-65 iirc. The pike is also 2-handed with very slow attack speed.