I visited this WHS in January 2018. Even if you have a couple of hours to spare in Singapore, this is an easy tick (even though Singapore as a country deserves a longer stay).

The Botanic Gardens are open daily from 5am till midnight and are a great half day or day trip in Singapore if you want to explore in more detail. There are 4 gates to access the Botanical Gardens: Tanglin Gate, Tyersall Gate, Nassim Gate and Bukit Timah Gate. If you intend to cover all the botanic garden use Tanglin and Bukit Timah Gates as you points of entry/exit. There are UNESCO WHS inscription plaques proudly displayed at Tanglin and Nassim Gates.

The gardens are free of charge but to access the National Orchid Garden (open daily 08:30 to 19:00) which is the core of this WHS you'll have to pay 5 dollars (it's definitely worth it!). With over 1000 species and 2000 hybrids, the National Orchid Garden is the product of the Garden's orchid breeding programme which began in 1928. Near Tanglin Gate, you can actually see scientists, botanists and horticulturists working in the labs. The Garden offers a permanent showcase and the largest display of orchids in the world. Some parts of this area are currently closed but it still is well worth the entrance fee for what you get to experience.

Established in 1859 by the Agrihorticultural Society, the Singapore Botanic Gardens were designed by Lawrence Niven, whose work reflected the English Garden style that influenced the emergence of numerous English landscape gardens and public parks from the 18th century. This layout as well as the historical buildings such as Burkill Hall (1868), Ridley Hall (1882), E. J. H. Corner House (1910) and Holttum Hall (1921) remain almost entirely intact.

With over 4 million visits reported annually, Singapore Botanic Gardens are the most visited botanic gardens in the world even though they didn't feel as crowded as other botanic gardens I have visited (probably due to it being huge in size and due to the fierce internal competition with the Gardens by the Bay). We really enjoyed our visit not only for the beautiful orchids but also for the unexpected birdwatching and wildlife treats (we effortlessly spotted black-naped orioles, sacred kingfishers, sunbirds, red-legged crakes, squirrels, skinks, butterflies and the elusive oriental whip snake).