There are plenty of incredible zoos around the world, and that is why deciding which one is the best is never easy. Some people look at the number of animals, while others focus on conservation work, visitor experiences or how closely the habitats resemble the wild.

But there is one massive animal attraction in Asia that animal lovers often say beats anything else out there. It is home to more than 4,000 animals, attracts around two million visitors a year and has become famous for its rainforest setting. Many visitors consider it to be the world's best zoo because of its size, open design and commitment to animal welfare.

The zoo in question is Singapore Zoo. Opened on June 27, 1973, the attraction started with around 300 animals from 70 species.

Today, it is home to more than 4,200 animals from over 315 species, with around a quarter classed as threatened or endangered.

The zoo covers 28 hectares on the edge of the Seletar Reservoir within Singapore's Central Water Catchment area.

What makes it stand out is its open-concept design, with animals kept in large habitats designed to resemble their natural environments.

The zoo also installed hidden moats, glass barriers and thick vegetation to separate visitors from the animals. At the same time, the owners of the attraction made sure that it’s still easy to spot the wildlife.

Over the years, the zoo has become famous for its conservation work and breeding programmes. It achieved a world first when Inuka the polar bear was born there in 1990, becoming the first polar bear cub born in the tropics. In 2009, it also successfully hatched a Komodo dragon outside Indonesia.

Among its most popular areas is Fragile Forest, which is a huge biodome where visitors walk through a tropical rainforest environment alongside free-roaming animals including fruit bats, lemurs and sloths.

Another highlight is Orangutan Island, where the zoo's famous orangutans swing above visitors.

The zoo is also home to African lions, cheetahs, white rhinos, chimpanzees, pygmy hippos, kangaroos, penguins and Komodo dragons.

Visitors can take a guided tram ride around the grounds, watch daily animal presentations, hand-feed selected animals, and even enjoy the zoo's popular Breakfast in the Wild experience.

So it can’t come as much of a surprise that tickets are among the most expensive zoo admissions in the world.

Standard adult tickets for non-residents cost S$49 (around £28.50), while children's tickets cost S$34 (around £19.80). Despite the prices, many visitors believe the experience is worth every penny.

One person wrote on TripAdvisor: "Best zoo I've ever been to. So many animals, so much space and very informative sessions."

Another added: "Absolutely best zoo on the planet. We had a backstage encounter with the elephants, and this was beyond the highlight of our trip. The staff were so wonderful and kind, and spent so much time talking to us about their beautiful elephants. We would recommend our amazing experience to everyone."

A third wrote: "The Singapore Zoo is one of the best zoos in the world, and I have been to many. As long as you are able to tolerate the heat and humidity, seeing the animals in the 'open' zoo is such a treat."

Someone else commented: "Expensive but definitely worth it. The size, the attractions and the kindness of staff absolutely justify the ticket price."