I have had my boxer-mix for eight years. I love her. She's been through everything with me. When I first got her, I was concerned about having a large dog and protecting her hips from going out on her. She's a couch potato, and the ups and downs were likely to wear on her joints.
The first food I bought for her was Purina One Large Breed Puppy food. Her skin was/is very sensitive. I noticed that her skin got flaky if I fed her (non-Purina) food at a friend's house.
When we'd get home, she'd load up on her regular food, and her standard sheen would return. Flakes gone.
After her first year, we continued with Purina either Large Breed or Lamb and Rice formulas (whichever we could find in the largest bag possible). We now have two large dogs, and if you don't get the monster bag - you'll be back for more all too soon.
On several occasions, I've skimped (I was in college...and times got tough) and bought the next cheaper bag of food. FAIL! Within a day, her fur would dull and skin would flake.
I am Purina loyal, and if you have a dog whose fur is dull - I suggest trying this brand.