If you have a melee group, go with a bard. If you have a caster group, go with a Chanter. If you want, you can do both since their requiem slow stacks with the enchanter's dreary deeds slow. I would chose an enchanter over a bard personally since they have so many enhancements and procs to other classes including their runes which keep them alive when pulling, twincast, chromatic haze, auras, etc. And if you wanted to avoid something like a spin stun while pulling, you could pet pull. If I had a 3 box caster heavy, I'd probably do chanter (puller with wiz merc) and mage (pettank with healer merc), and consider these with another wiz merc:
Beastlord - mana, buffs, and extra pets in case your mage pet falls, and run speed and FD if the group is gonna wipe (non-caster gear)
Necro - pets, undead slows, FD, and the odd evac (cons - really need epic and robe to be good)
Wiz - straight nukes and portability, great burns, plus the evac when things go wrong (cons - need gear for mana)
Shaman - still... pets, with heals, big ol' slow, buffs, cheetah speed, rez/CoW etc. (cons - need tons of gear and AA to be amazing)
Druid - eh... they have a ton of great buffs, ports, nuke-heals, rez/CoW, etc but their dps won't really compare to what you get with one of the above (in my opinion)
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My original box was Enc, mage, Wiz but as the content got harder I had to swap the mage for an SK. Too many named who spawn adds which killed a group member or healer before they got locked down. I did Enc, SK, Wiz for a long time but struggled with some of the RoS named. I just added a shaman which really helped. I'm swapping out the Wiz for Mage or BL now and again just for fun.