I had polar bears just once. Never again. You know why 
I actually forgot tapirs were in the game until someone recommended putting them into a mixed habitat with anteaters, which sparked my imagination.
I haven't used bengal tigers since the beta. No idea why. Think it's a subconscious thing - they do all look pretty similar and I'm after that attachment to the animals in my zoos, over numbers.
What makes it into each and every one of my zoos:

I actually forgot tapirs were in the game until someone recommended putting them into a mixed habitat with anteaters, which sparked my imagination.
I haven't used bengal tigers since the beta. No idea why. Think it's a subconscious thing - they do all look pretty similar and I'm after that attachment to the animals in my zoos, over numbers.
What makes it into each and every one of my zoos:
- Cassowaries (gimme that Shoebill DLC, c'mon Frontier)
- Alligators
- Cheetahs
- The tortoises (because I like having them walk through that wall piece with the round hole in the centre, which I lower into the ground a bit).
- Pygmy hippos (the A rank animal of cuteness and habitat building enjoyment)