depends a lot on child, your expectations, and schools available.
ds is quite clever. he could do well academically, but he is also very peer driven (if it isn't 'cool' he won't do it) so we looked for a school where being bright and doing well was an important part of the culture.
he is actually probably going to go to the grammar school (we will know in march)
he also likes sport, but is no future England player, so a school where he would get lots of sport for fun/team experience was important too.
dd is musical, and very clever in 'non girls' subjects- especially maths she is also quiet in the class and tends to get overlooked. She is only year 4, but I think we will try and get her into the girls school.
My friend's dd is not really an academic type. But she loves her music and drama. They have looked at a school with a big emphasis on performing arts, and at the same time good solid results.
Another friend has a daughter who wants to be a doctor, they went purely by the highest results on the league table.
We visited loads of schools, atmosphere, behaviour and how well the staff interacted with parents and kids was important too.