Since you're playing Revised Ranger, I'd take advantage of their ability to choose all humanoids as your favored enemy. All humanoids is such a broad category that it's typically the best one unless you know you're going to be fighting something else way more often.
That being said, undead is also a very strong choice for Curse of Strahd and it probably provides more "flavour" than having humanoids as your favored enemy. If you're only concerned about utility/strength with this ability, humanoids is probably your best bet, because humanoids covers such a huge blanket. Undead is a close second though.
Greater Favored Enemy will be a much less useful trait in this particular campaign however--I think the only choice that really has much of a presence is Fiends, and they're pretty sparse (I could be wrong about that though).