Questing/grinding always works best with classes that can tank, because they wont wipe when they pull a bigger group and the wont kite a mob into another group it even works better with classes that can tank and heal themselve
That means:
1. Palas, dudus, monks with Tankskill
2. Dk, warri with tankskill
3. rouge, Hunter (tankpet)
4. all the rest, since range classes can`t really survive alot of damage and CCing mobs is kind of difficult for a bot.
BGs work best with almost every heal class or a Rouge till lvl 30, from lvl 30 on a hunter should be the best class for it atm.
Farming: Dudu becauseit can gather herbs in travelform.
Dungeons: melee DD classes. Heals and casters sometimes have the problem to be Oos of the target.
It really depends on what you want to do. My Farm account has a dudu, since it can level from 1-90 staying alive most of the time, has the travelform very early and can gather herbs faster.