When it comes to farming, Druids and Death Knights are usually the best. Druids can get between nodes quickly due to shape shifting. Death Knights can walk on water, wear plate, and blood is self healing so they haven't much downtime between mobs. The most OP in terms of damage, not a huge difference between any of the classes.
Warlocks. My Demon warlock easily knocked down 5 mobs + an elite at a time. I expect the Warlocks to get a huge nerf hammer on next patch. If u haven't tried the Warlock yet try them now and watch your foes fall down 5-8 at time before Blizzard nerf them. Super fun and you will thank me.