As you've probably gathered, there really aren't that many, and are geographically spread across London, but at the top of my head from past research, I would have said Alleyns, Latymer Upper, Highgate, Kingston Grammar, Ibstock. For the right profile child, Emanuel is more appealing. Likes of Westminster, KCS etc take girls from sixth form.
I think you need to did a little deeper for extra curricular stuff. Music tends to be a decent standard in most. Depending on whether you have a girl or a boy, sports can vary. Feedback can be x school has very good sports, and then you realise that they provide 3 x more fixtures for sport vs the girls...so my suggestion would be to look closely at the schools' sports websites, number of fixtures, how many teams, who they play, results etc if they can be bothered to update them. Depending on how much interest your DC have in sport, this can change your view on defining the 'best'...music and sports were a key factor for us after academics.
I am assuming your question was around 11+ entry. I cannot comment which is the best per se, as my experience is only with one I've named but being realistic, it's tricky to get one that serves all the best but if you they do them very well, and continue to try and approve on their shortcomings (and probe the Head of Sports or Music more on the detail behind the open day pitches) then this is what I would recommend going forward.
Good luck!