Act 2 nightmare offensive mercs make the best mercs for barbs in general, since they use the Might aura. They can also use Doom poleaxes, which gives them the aura Holy Freeze, though that's more of an upgrade for the rich.
Actually, my lvl 84 conc barb is having bigger AR issues than he is damage issues. Until I can get more AR gear and level up some more, I'm using a hell act 2 offensive merc (blessed aim). He bumps my AR up to 10k or so, which is crucial. Without him, I whiff a LOT. I do plan on getting a nm act 2 offensive merc eventually, though.