Shaman
Healing, Totems, Bloodlust

ST--[------7---]
AOE-[------7---]
CC--[---4------]
SUR-[---4------]
SYN-[---------10]
STK-[--------9-]
MM--[---------10]
ALL-[------7---] - 51 Total

Shamans are the cliché multibox class for obvious reasons. They stack well, are easy to use, and can heal the group.
If you are new to multi-boxing this is probably the class you want to use the most of. They are easy to setup, and stack well.
Their DPS won't be what a pure DPS class is, but that’s the tradeoff.
Their AOE must be targeted.
Bringing additional shamans means totems, TOtems, TOTEMS! Bloodlust rotations, and multiple chain heals.
The more chain heals you have casted at once, the more your group gets healed.

Mage
INT Buff, Portals, Food/Water

ST--[---------10]
AOE-[---------10]
CC--[------7---]
SUR-[--3-------]
SYN-[----5-----]
STK-[1---------]
MM--[---------10]
ALL-[-----6----] - 44 Total

Mages bring an AI buff, as well as food/water to the table (literally).
Mages are glass cannons, point them at a target, and they unleash hell until they get killed.
They also have the best AOE in the game (arcane explosion) It requires no targeting for Aoe.
Bringing more than one mage won't gain you additional buffs. They are however the highest ranged DPS class in the game. That means lots of DMG.

Warlock
Soulstone, STAM Buff (w/ Imp), Curses, Pets

ST--[--------9-]
AOE-[--------9-]
CC--[----5-----]
SUR-[--3-------]
SYN-[-------8--]
STK-[-------8--]
MM--[------7---]
ALL-[------7---] - 49 Total

Warlocks bring a STAM buff if their IMP is out, as well as Soulstones.
Curses have various effects on the enemy, the more warlocks you have, the more curses you can apply.
Like mages they are glass cannons. Their aoe however requires targeting to use effectively.
Bringing additional Warlocks means more stoulstones, more pets, more curses.
Although not as much DPS as mages, stacking locks will be a very high DPS group that has a little more versatility.