As the mother of two children, I consider myself as a bit of an expert about schools and schooling. Before you select a school, a good exercise to do is to put your expectations from a school down on paper. Take a hard look at it. Mark the ones you simply cannot compromise on; ones you are willing to consider a compromise; ones you wish they had but won't affect your selection.
If you already have a list of schools see them on a map visually. Do you plan to move close to the school or are you alright with a slighter long commute.
Distance is always a factor and in the child's interest, it is better school is a short commute away.
Which board? ICSE has a in-depth syllabus with subjects like Science being split into Physics, Biology and Chemistry quite early on and different exams for each. CBSE has 6 subjects and children who like Math and Science over Social science and English may enjoy this syllabus more. Both boards thrive on the rote system.
IGCSE board schools focus more on independent thought and less on a rote system.
The new age schools across boards have several co-curricular activities lined up for children. Alternative schooling with a more open system of teaching is also available. Several parents prefer homeschooling as well. It is important to see which kind of school would best suit your child's temperament and what fits your budget.
Make no mistake private schooling is a long-term expense that only gets bigger as they child moves higher up.
If you are looking at high personal attention look for classes with smaller number of students.
Do ask around about the school, it always helps to get feedback.
Finally, there is no best school just a school that suits you best.