At Madras cafe at CP or at ito near MCD u can find best South Indian
Kerala restaurant in green park
Sarvana Bhawan is cheap and good
Carnatic Cafe, and if you want to try at road side then there is place in RK PURAM SECTOR 5 near Tamil Sangam named Mathew's Cafe which provide best South Indian food in Delhi at affordable prices...
Shree rathnam is wonderful
South Indian hut. These shops are available in delhi in various locations. Price is very reasonable and food is fresh.