Druid feral/guardian is pretty fast and my favorite by far. Sprint, stampeding roar, skull bash, and wild charge get you all over an instance pretty fast and you can ranged pull with faerie fire. You have pretty much unlimited high output aoe with berzerk+swipe in cat form, when the instances get higher level (60s, 70s) you can switch to bear if needed and thrash/swipe. Since 5.0.4 you can now break roots without using your sprints, which helps in places like BRD with those damn nets.
Mage aoe output is also high and unlimited but it's harder to tank really large pulls being in cloth.
DK as frost has good aoe and ranged pull, but there are cooldowns and no way to switch to tank if necessary. Blood DK can tank anything but aoe dps is lower. There's also no way to speed up travel time in the instance due to no charge/sprint and no way to break roots.
Warrior has good mobility with charge and heroic leap and such, and tank spec can put out good aoe damage, but you can't sprint.
Paladin was my favorite before I tried druid. May still be good with mass exorcism and such but overall I don't think they have constant high damage aoe like a mage or druid.
Rogue/hunter/warlock I don't know.
Shaman suck.
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