It depends on what source you are asking :-) Hardiness zones are really not a very good reference point for a sub tropical or tropical climate as they deal only with average minimum low temps. There is really no difference between Singapore's low temps and high temps - January is considered the coldest month with average temps of 26C and February the hottest with average temps of 27C - not much of a distinction!! Heat zones and precipitation averages would be much more meaningful in your situation but they are not charted or recorded in the same manner as hardiness zones.

Sources indicate Singapore is listed as a zone 12-14. But just letting folks know that you live in a tropical climate with NO change in seasons and subject to monsoonal rainfall should be sufficient :-)

Thank you for the help!