Personally, I find the wood pieces of the arctic pack quite useful and a polar bear is a polar bear and I personally love the reindeer, so yeah, if you like to build, that's a good pack.
The African pack has a lot of versatile pieces too, so also a good pick in my book.
If you're less of a builder and more of an animal collector, I think the north american pack is a must, especially if you want to have low budget zoos. But looking at your current roster that might not be your thing?
With the europe pack I absolutely love the broken wall decals, but in my opinion most of the other pieces are way too "Amsterdam" and less "Zoo".