Hi YS,
As mentioned by davidasaph, there is no official ranking of
International Schools in Singapore. Deciding which school is best for your child/children will always be a subjective exercise, since the criteria for what constitutes the "best" schools can vary widely from parent to parent. Some may be more interested in the academic performance of the school's students on standardised tests (e.g. A-levels or IB Diploma), while others may be more concerned with finding a school that offers small class sizes and a family-like environment. Ultimately, you'll have to make your own decision about which school is best based on your own set of criteria. Of course, if you can find like-minded parents whose opinions you trust, then their input can definitely be useful in helping you decide.
While there are certainly a few big-name schools that most people will hear about as soon as they start looking at
International Schools in Singapore, it's worth noting that there are (at last count) 29 international schools here offering schooling in English. While these schools cover a wide spectrum in terms of the programmes and facilities they offer, it may be helpful to familiarise yourself with these myriad options to help you develop a short-list of schools for further research.