As an Atlanta native, and a person who spends way too much time in the Atlanta airport, here are my favorites;
One Flew South - Terminal E
Chicken+Beer - Concourse D
Ecco - International Terminal
Varsano's Pizza - Concourse A
El Taco - International Terminal and Concourse C
TAP - Concourse A
Paschal's (Good Southern Soul Food!) - Concourses A, B, C
Three Taverns - Concourse A
Food Network Kitchen - Terminal D
Grindhouse Killer Burgers - Concourse D
The Varsity (An Atlanta tradition and owned by some old friends of mine) - International Terminal - Order two walk-a-dogs, a bag o' rags and a FO. That'll get you two chili dogs, potato chips and a frosted orange (sort of an orange milkshake) to go.