Horde is the most powerful for several reasons:
Goblins still arguably possess the technology created during the Garrosh era - all this talk about Dwarven steam tanks and everyone seems to forget the giant metal scorpions with missile attachments and sawblade hands or the assortment of long-range artillery those apparently oh-so-crude and primitive Orcs have at their disposal. Orcs are bound to be more numerous now with the addition of Outland and Draenor to the universe, they're not a small group of refugees freshly crashed on Kalimdor anymore. Further, I haven't seen anyone mention the giant gun in the middle of Bilgewater Harbor with its sights set on Stormwind. Goodbye, Stormwind Navy. How many hits can Ironforge take with that thing?
Forsaken plague has also been brought up, it goes without saying this would be devastating to the Alliance. Of course, there is the whole flying space ships of the Draenei to contend with - in my opinion, the only serious weapon against the Horde. But good luck getting them to use it against cities filled with civilian populations, and enjoy dodging mana bombs and suicidal zeppelin commanders. Its cute the Alliance think they stand a chance behind their teary eyed child king though.