@shailendra, Santa Maria is doubtlessly the best. Teacher -child ratio is far better than any other school, they calculate age by 31st September not March, no other school does that. Their fee structure is nominal unlike most south delhi schools. And I have personally seen the principal bring very open minded and positive towards little children who just walk into her office and talk to her directly - again unlike most schools where principals are too scary. Despite being Christian minority school they do not over emphasize the religious quotient - as in case of mount Carmel etc, also they take enough children from non-Christian backgrounds to enable a truly holistic development.
Although all schools are good, and a each child is different, I'd say go for Santa Maria without thinking twice if you're lucky enough to be selected.

I hope this helps.