The paladin and shadowknight mercs are crazy tanks. They can literally powerlevel you through content and dungeons up to 80. The assassin merc will make you cry with it's dps. The inquisitor merc is the only heal merc that actually knows what it's doing. He wont mess around getting rid of debuffs and healing you, but I found it hard to keep him alive if he participates in combat inside dungeons. Just stick him on follow and enjoy the 5 group buffs or so and let him herp-derp it up as he heals/cures you all day long.