Najafgarh Jheel or Najafgarh Drain (its sad its called a drain) or Najafgarh Lake located in south west corner of Delhi is a great place to see flamingoes in large numbers. Apart from flamingoes other water birds are common in this area.
Najafgarh Jheel can be reached easily via a well metalled road along the canal or drain from Dwarka. From NH-8 go towards Najafgarh along the south side of Airport. From there when you hit the canal or drain , take a left turn and continue for 10 kms south along the banks to reach your destination. Put this coordinate in maps (28.505473, 76.943023).
Have full body cover as there are lots of thorny trees along the banks and chances are they will hurt you. Carry water and snacks as there is nothing in the vicinity.