I would like to see the Dwarves and the Misty Mountains factions released first after the demo, because they are the most diverse factions, have, IMHO, some of the best designs in the mod, and contain my favorite heroes (Thorin, Kili, Fili, Smaug, The Defiler, Bolg, and others) and two of my favorite towns in Middle-earth (Dale and Laketown). So my vote goes for them.
However, I would like to see the other factions Imladris, Lothlorien and Angmar released soon, too, as they also have some great stuff (fingers crossed we will get back the White Council Chamber
).