I will say Aqua because it is the most likely possibility. As far as we know, she has the right Keyblade to access Ventus in Castle Oblivion.However, there's always the possibility that Sora can (somehow) be given the Master Keeper while Aqua is in the Realm of Darkness, just as the Trailer depicts. At that point, Ventus will probably be the most likely to awaken first.But I honestly don't know about Terra.