The Crazy Waiguorens - Alterac Mountains on CN1 (奥特兰克) has the busiest guild at the moment. Our PVE server guild, TCW, on CN5 Arygos (亚雷戈斯) is pretty quiet as everyone seems to be taking a break until the new patch comes out. Both are Horde guilds.
If you dislike being ganked on Timeless Isle over and over again, Arygos may the better choice, especially given that Normal and Heroic SoO will be available cross-realm shortly. However, there are usually many more people online on Alterac, so its much more social at the moment. Pity we can't have a cross-realm guild really.