Which is better: torch-on, liquid or cementitious waterproofing?
None is best everywhere. Torch-on modified bitumen sheets are robust and quick on large, simple roofs but involve open flame and many laps. Liquid-applied membranes are seamless and good around penetrations and complex shapes, but depend on achieving the specified thickness. Cementitious membranes suit wet areas under tiles and damp substrates. Choose for the substrate, exposure, detailing, access and maintenance plan, and follow the manufacturer's system.
From the RRD Field Guide, checked against the sources cited in Flat Roof and Wet-Area Waterproofing in Singapore: Membranes, Ponding, Leak Tracing and Repair. General information, not advice for your project.
The fuller picture
Most flat-roof and bathroom leaks in Singapore come from failed membrane details, not the open field: upturns, outlets, penetrations and laps. Diagnose before repairing, using a controlled flood (ponding) test, moisture mapping and inspection. The code of practice is SS 637:2018. In strata buildings the roof membrane is common property, so the MCST repairs it.