Nm merc have the best skill out of all mercs. Hell mercs have the same skills as Norm mercs but they are higher level.
Nm act 2 might merc is the best because he gives you a good dam bonus, and they are fairly easy to level.
My zon doesnt have a valk skill as i preffer to put points into donge/evade/avoid. She uses act2 nm merc at lvl 72. Took me 3days to get him there from lvl33 when i bought him. All in all it took about 8hours or so total to get him to 72, he levels up quick still. Anyway I have on him shaft with p-amethist in it, crown of thives with 11%LL, and Ith war pike (3*ohm, 3*40/15ias). He rules hell cows, he rules hell diablo, he rules hell bhaal, he is a fuc ken tank!!! The valk cant even compare to my merc, so not only did I save skill points, I also get huge dam bonus from might.
I also have the same merc with my MF barb who even though uses 2*ith sword is very weak. Thats what getting setup for mf does to you. Both swords are 180mf, he is weak alone but with might bonus from merc he still rules!!!