Well, it depends on how broadly you're willing to use the concept of rainforest. After all, the actual rainforest near Singapore Zoo isn't exactly top quality, primary rainforest (exactly as one would expect in a small and densely inhabited nation). Even if using a broad definition of the term, it's true that there are no African zoos worth a mention in rainforest regions, but there are a few in South America. The Atlantic rainforest region includes much of southeastern Brazil, including both Rio de Janeiro (decent rainforest in Tijuca) and São Paulo (rainforest remains in Serra da Cantareira). Most other South American zoos worth a mention are far outside rainforest zones, but Cali Zoo is in the Chocó-Magdalena, although the remaining forested sections in the Cauca River Valley are tiny to say the least. Regardless, even the best South American zoos don't really match Singapore. The same can be said about other southeast Asian zoos in rainforest zones. There are no world class zoos in Central America and Australia's top zoos are all outside the tropical rainforest zone of the country.
That was the "easy" part. It becomes less obvious when remembering that rainforests aren't a strictly tropical thing. There are large temperate rainforest zones in the Pacific Northwest of North America (Woodland Park is probably the best in this region), southeastern Australia and elsewhere.
I would say that it is true that Singapore is the best "tropical rainforest zoo". If including zoos in temperate rainforest zones, it's more open for discussion. Regardless, I'm not sure this self-proclaimed title is all that saying or accurate. Singapore is definitely a world class zoo. Rainforest zone or not.
The "open" concept you mention where fences/barriers are hidden is used widely in many zoos around the world.
