Vigilant of Stendarr, because just like in vanilla Oblivion there is no real faction for a religious character (cleric, paladin, priest, monk, pilgrim, crusader, etc.) Which leads to my other choice: an Imperial Cult or Knight of the Eight faction.
I don't mind implementing the other factions for other players, although I wouldn't really play them or spend money on them if they were DLC since I only really play goodie two-shoes. Joining a group of Vampire hunters would be awesome though