Singapore Zoological Gardens
The Zoo takes great pride in having no fences, no bars, no restraints for animals. Most of the habitats have 'psychological' barriers instead like moats of water, raised rock platforms and walls, or just enforced feeding areas. The primate collection has largely free reign of the entire park and you often spied a monkey in a nearby tree.
Some of the habitats were enclosed simply through nessesity. The cool enclosure for the penguins, for example.
The zoo had also taken great care to simulate habitats that are as natural as possible. The cheetahs had grass and trees to climb in, the baboons had dust and rocks to congregate in. I didn't see one animal that looked remotely upset or bored.
We spent about 4 hours at the zoo, walked our feet off and had a ball. An absolutely stunning place, and a photographers paradise.
My favourite of the day would have to be the white tigers.