Best in term of what

Curriculum?
Teacher's qualification?
Facilities ?
Student's ratio
Student's academic (test) achievement?
Inclusiveness ?
Extra curricular activities ?

Frankly, it is hard to quantify most of the parameters above. That's why many educators is opposed to the idea of constructing a ranking for school as it could be very misleading and potentially do more harm than good.

I believe the better measure should be: which school is a better match for a particular children's education need . For example, Shafalla and Sunbeam may not have a top academic reputation, but for student with learning difficulties they may be the best school in the country. For students aspiring to take university entrance exams in Philipine or India, the American School may not be the best option despite being much more expensive compared to those offering Philipine or Indian national curriculum. The list goes on

Just my afternoon rambling.....