The Aura Archer
The idea here is to have an act one bow merc hanging out in the background providing big hits and giving aura buffs to everyone in the group. The act one merc is great because their AI keeps them behind you so they stay alive longer and don't die to curses like melee mercs do. Did I mention you get level 15 might aura from Lastwish too? They're fun to use with almost any build too.
Type: Act 1 Bow Merc
Weapon: Faith Bow (Shadow bows are my preference because of the high base IAS and the spread of min-max damage)
Helm: Dream or Andarial's Visage (Dream is nice because of the extra direct lightening damage and the little "pulse" damage, otherwise use whatever you want)
Armor: Fortitude or Dragon (Again, the extra damage of Fortitude makes her bow damage great considering you already get Fanaticism from Faith)
This merc will stand in the background giving everyone in the group Fanaticism, hit for about 5-8k, and hardly ever dies. You can have her hit bosses like Meph from across the moat and go afk while she does some killing if you stand at the right spot. For chaos runs she's the preferred build since she doesn't die to curses. Pick the cold arrow version because she will slow enemies she hits too.