While there are many NGOs in Delhi, there are few things you must take care of before choosing the best NGO. An NGO in India is categorized into A Trust, Section 8 company, and a Society. Whichever Ngo you use must be registered under any of these Acts, as non-compliance to these act registrations can lead to penalties to the Ngo.
Moreover, if you wish to donate money to these NGOs you will be eligible for tax exemption on that particular amount. But for this that NGO must be registered under the 80G & 12A which makes the Ngo and the donor eligible for Tax exemption.