We are considering getting an annual pass for the family either for the Zoo or the Gardens by the Bay or the SEA Aquarium - we have a six year old daughter who loves animals and plants. Which one would you recommend?

Zoo! As they have different animals! Best to get the combination of Zoo, bird park and night safari and river safari. It's cheaper and can explore more and different things. Garden by the baby and sea aquarium, i feel it's quite constrain hence won't see that long too. I visit zoo more often than sea aquarium/garden by the bay.