I am not going to give you one because like trying to pick one favorite song when there are so many genres of music ther are so many types of restaurants, but here are two greats.
Pizza-Fellini's- It's great food (Fresh, natural), great atmosphere, a number of locations (On Ponce, on La Vista, On Howell Mill) and cheap, also if your interested, cute waitresses (A generally friendly staff).
General Good Food Place -Pangaea- It's a little out of the way (Huff Road, which is a road off of Howell Mill Road right after 14th street) but it is worth finding this place. It is very odd, eclectic food, once again no dress code and pretty cheap.
Have a great time trying these two places and it will cost you less than 25$, even if you take a friend.
Oh yeah, I also always liked to grab lunch at the Whole Foods on Ponce.