Well I think I would agree that PQs are probably the best way to rank up the guild based on;
i, A significant portion of the xp you get from scenarios is bonus xp at the end, which is it seems not counted.
ii, Open RVR xp is low.
iii, PVE mobs don't suffer from diminishing returns.
Probably need to keep the mob levels high enough and the groups small enough to get decent xp. 2 or 3 people's probably best. Fun times if you can bear the grind.
Should be noted that they nerfed AOE xp somehow a way back. Not sure how, or if it'd affect grinding like this.