I've visited Delhi twice in the last three months, and while there I've discovered what is now one of my absolute favorite "dhabas". Rajinder Da Dhaba, while a little big for a traditional dhaba and often somewhat crowded, offers outstanding food. Located in Safdarjung, I would often order from them at 5pm (their opening time) and receive my delivery at AIIMS main campus by 5:30 or 5:45, which is a miracle on a Delhi weekday. My personal recommendations are every single appetizer on their menu, their chili paneer, alu kulcha/parantha, and their biriyanis. Like most dhabas, the biriyani is a little light on meat but the flavors are so excellent that it is well worth it, served in a clay pot and rich with butter and black pepper. If I could go back right now, I would buy Galauti Kabob, Achari Chicken, Malai Chicken, a veg biriyani, and two orders of alu kulcha.
Rajinder da dhaba used to open all night back in the days but since last few years they have started closing just after mid night because police wont allow them to open all night any more. Their food is excellent and I have been going there for as long as I can remember. It used to be a place me and my friends would meet 12ish to have a few drinks there. For those who don't know, it is not a pub and people used to just park their cars and drink inside their cars and food used to be served in their cars. Those days are long gone.