Alliance is usually chosen more often then not by newer players, and the fact is that 90% of people on the Aliiance are 10, and therefor, really really realllly annoying.Horde is more organized then Alliance and that is why in the majority of battlegroup horde wipes the floor with alliance.
I play Alliance =p
Best Dps would have to be a rogue or a mage
There's no best tank, the druid, pally and wariror can tank equally well with their own upsides... those being..
Pally: You really only have two choices with this class... Tank or Holy, since you can't dps worth Sh!t.
Warrior:Warriors are always expected to be tanks, end of story
Druids: Druids can do just about anything and be good at it, their main downside is their dps, but even that isn't half bad
The best healer would have to be the priest but both the shaman and the druid in xcel in places where the priests has epic fails.