I have 3 Shepherds; two are fed pretty much the same:
1.) Brand of food -
Acana Grain Free (we rotate between the chicken, lamb and fish varieties)2.) Why that particular brand -
I have a high level of trust / confidence in the company (Champion Pet Foods) and the dogs do well on it3.) Age of German Shepherd -
Male (83 lbs): 9 yrs, Female (63 lbs): 3 yrs4.) Frequency of feeding (once a day, twice a day, three times a day, etc .....) -
3 times per day (about 1 cup each time) so that they don't have a huge volume to process. They're good eaters and always motivated by food. 
My old girl gets:
1.) Brand of food -
2/3 of her diet is home prepared (raw), using a mix (See Spot Live Longer) plus Carnivora (Urban Carnivore) veggie & fruit mix; the other meal is whatever kibble the other 2 are getting at the time2.) Why that particular brand -
The old girl has a sensitive tummy; can't eat kibble for every meal now. She loves her food, does well on it and it's easy for me. 3.) Age of German Shepherd -
11.5 yrs4.) Frequency of feeding (once a day, twice a day, three times a day, etc .....) -
3 x per day (2 cups home prepared; 1 cup kibble)