DK tank - workable. Definitely not a hard tank to use in multiboxing. Just need to use the pesky click in next window mechanism.
WAR tank - wouldn't even dream of tanking as a warrior running 5 toons at once. Have done so with just 2 toons and it sucked as bad as it sounds. Icecrown heroics for example when they came out. Yep, my blood pressure ran high.
Ele shaman - a staple in the MB community. Great knockback, extra totems, easy AOE spells that require no pesky click in next window mechanism.
Hunter - a toughie to multibox due to the IWT annoyance. They won't IWT like every other class in the game can due to the melee vs range mechanics.
Moonkin - No comment on this one. I don't play druids.
Warlock - Can be good with the right synergies. Thinking a spell haste + dot / haste build in general here.
Mage - another multiboxer staple. Very easy to play, frost nova drives people up a wall. Can port and give food. Another great set of things for a multiboxer.
Holy Priest - People will probably disagree but, I would not use this healer for anything but a melee team in PvE. Squishy and not as reliable as a chain healer in range groups.
Resto shaman - pretty much the PvE healer for multiboxers.
Resto druid - see boomkin.