Chandni Chowk is the best place for street food in Delhi.
You have to go on street-food trail of CR Park and Gole Market as they're a street-food haven in the city.
Matia Mahal in Old Delhi is a lane filled with street food options as is all of Old Delhi. You can also explore Nizamuddin!
Nizamudin dargah street would be another suggestion to list
Chandni chowk ,jama masjid and old delhi market are the best places in delhi for street food.