figure out what you want to do first, it hugely influences what classes to pick. world pvp, arena, battlegrounds, regular instances, heroics, raiding, farming gold, dailies, questing, ?

for example I multibox to run heroics solo and to gear up 1-2 of my chars so they can raid. so i have a tank (paladin, but DK works great too), a healer (resto shaman, priest or druid are also fantastic), a mage (porting/food/dps), and 2 ele shamans (totems, lust, heal if needed). i do a little bit of everything in wow, but my team is heavily geared towards running heroics.

if I wanted to mostly do world pvp or dailies or farm gold I'd pick 5x hunters or 5x boomkins, they work out great. no clue for arena, it feels very multiboxer unfriendly atm with the burst damage of melee.