I use Mage because pet is awesome sure beats warrior merc at higher levels valley for example tier 5 journeyman merc gets owned easily. With mage and Cleric merc then second account I say go for Shaman I made a Beastlord not really impressed pet is lacking because always a good 10 or 15 levels behind until hit higher levels, besides Shamans have second best dot behind Necro, they can slow, heals are decent and can some what tank in a pinch beats having Mage attempting it
LOL
then I would use Rogue or Wizard merc. Right now I am focusing on grinding merc AA which is a slow task but I believe it will help out in the end I am in no rush I am at level 96 so figure next 6 months get merc some AA and gear then can goto to 100 or 105 and things will be a little easier going from there because sure the higher end mobs will need a better merc for healing and dps.
I do wonder do Mercs benefit whenever there is a double xp week......