There is no 'best food' other than the food that best suits the individual dog. Some people prefer to feed their dogs a raw diet, some prefer to use a commercially made food. Best I can say is avoid anything that's cereal-based (bulk in, bulk out) and go for one that has the main, first, ingredient listed as meat, or fish. And if this isn't a puppy, keep to a sensible protein level - around 24% is enough. To start with you could use chicken with boiled rice.
However- if you just found this Lab, if you keep her, that's theft. Take her to your local Shelter which is where people with a missing dog will be checking. Let lthem know you would like to be considered when finding her another home, if nobody comes forward to claim her. That way you'll adopt her legally, with the paperwork. As it is, if somebody sees you out with her, they are entitled to claim her back.