Which expansion pack should I get next?

As usual obvs, kinda depends on your play style and what you're looking for. I think all the packs listed have something to recommend them really. Based on what I would probably get, I picked City Living, as, like some others have said, apartments and festivals are things that make the sim world feel a lot more "real" to me, as it's been a big part of my own life. But, that might not be the case for you, and I concur that the Cats & Dogs and Get Together worlds are great additions as well. Personally, I think I use all the game elements from those packs listed from time to time, so I don't think there's a "bad' answer here, just go with what feels right to you!