I'm mostly interested in the animals, not so much in the building part. So for me the Southeast Asia Animal Pack was one of the best, which gave me a big bunch of animals! I've always been a huge fan of the babirusa since I first saw them in a zoo myself <3
But the very best would be the Aquatic pack mainly due to the new animations/mechanics. I love to see animals swimming underwater, it gives a lot of dimension to a zoo being able to make underwater viewing areas. One such pack that could create the same effect is a pack with birds. If that isn't a possibility, and the next DLC is just gonna be the 8 animal pack or a few animals/with building pieces, then i'd rather have another animal pack like the Southeast Asia pack.
I'd personally love an European Animal Pack with 8 animals in it, like the red fox, red deer, badger, wild boar, etc, animals I can encounter here in the wilds

(but also the European fallow deer even though I can't encounter them in my country)