Delhi's food is a product of its history — Mughal court cooking, Punjabi refugee cuisine after Partition, and street food traditions that go back centuries, plus regional food from every part of India.
Old Delhi street food is the essential experience:
Punjabi and North Indian standards — butter chicken, dal makhani, chole bhature, rajma chawal, tandoori breads. Delhi does these better and more consistently than almost anywhere.
Mughlai cuisine — rich, meat-heavy dishes descended from the imperial kitchens: korma, nihari, haleem, biryani.
Chole bhature — spiced chickpeas with a large fried bread, the classic Delhi breakfast or lunch.
Butter chicken and dal makhani — both credited to Delhi restaurants in the years after Partition.
Momos — Tibetan-origin dumplings, now completely naturalised as Delhi street food and available everywhere.
Regional food. Because people from every Indian state live in Delhi, you can eat South Indian, Bengali, Kashmiri, Naga, Rajasthani and Gujarati food at a high standard. Bengali Market, Pandara Road and the various state bhawan canteens are good hunting grounds — the state bhawan canteens in particular serve authentic regional food at low prices.
Practical advice for street food. Choose busy stalls with high turnover. Eat freshly cooked hot items rather than anything sitting out. Avoid raw chutneys, salads and tap water at informal stalls, and skip ice. If your stomach isn't used to India, ease in — a guided food walk in Old Delhi is a good way to eat well with less risk.