In Nizamuddin, which is a lot closer for me, and is an easy diversion on the way back from office, there are a few favourites -
1) Ap ki khatir - on the main lodhi road facing side, a unmarked shop - will have lots of people sitting in cars parked right outside it and eating away. For Kakori/Gilawati/burra kebabs. See Sab ki Khatir review as well on this thread for similar food, in a better location.
2) Nasir Iqbal - in the main dargah lane. For gravy items - standard all round restaurant, with proper spacious seating.
3) Karims - I prefer this branch to the old delhi branch. Not only does this seem neater, cleaner and spacious, the food seems to me to be better prepared here. Around the corner from Nasir Iqbal. Ask anyone in the dargah lane.
4) Moradabadi biryani - In the food lane just before Ap ki Khatir (and it really can't be classified as any thing else - its a lane with 40 restaurants packed next to each other) walk down around 50m and on the left you will see a restaurant called Moradabad ke Masoor Biryani - thats my favourite for Bade ka biryani. Ask him to get a plate of Bheja masala and nihari, and have them with the biryani - the bheja goes perfectly with it.
5) Ghalib kebab corner - For buff sheekh kebabs and mutton sheekh kebabs with Korma and rotis. This is in the lane which shoots off to the left from the food lane when you just enter it.